30th institute on lake superior geology lake field...

30th AnnualInstitute on Lake Superior Geology

FIELD TRIP 3

April 28, 1984, Wausau, Wisconsin

TH E WAUSAU SYE N TE CO M PLEX

WA U S AU

PLUTON

W AU S A U

N I N E M 1 L E

P LU T 0 Nmiles

30th Annual Institute on Lake Superior Geology

THE WAUSAU SYENITE COMPLEX

April 28, 1984, Wausau, Wisconsin

30TH ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY

FIELD TRIP #3


CENTRAL WISCONSIN*

by

Paul E. MyersDepartment of Geology

University of WisconsinEau Claire, Wisconsin 54701

Mohan K. SoodDepartment of Earth Sciences

Northeastern Illinois UniversityChicago, Illinois 60625

Louis A. BerlinDepartment of Earth Sciences

Northeastern Illinois UniversityChicago, Illinois 60625

Al U. Faister920 McIntosh StreetWausau, Wisconsin

.54401

* For sale by Paul E. Myers, Department of Geology, Universityof Wisconsin, Eau Claire, Wisconsin 54701 [$6.00]

3 0 ~ ~ ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY

FIELD TRIP #3

THE WAUSAU SYEN I T E COMPLEX

CENTRAL WISCONSIN*

P a u l E. M y e r s D e p a r t m e n t o f G e o l o g y

U n i v e r s i t y o f W i s c o n s i n Eau C l a i r e , W i s c o n s i n 5 4 7 0 1

Mohan K . Sood D e p a r t m e n t o f E a r t h S c i e n c e s

N o r t h e a s t e r n I l l i n o i s U n i v e r s i t y C h i c a g o , I 1 1 i n o i s 6 0 6 2 5

L o u i s A . B e r l i n D e p a r t m e n t o f E a r t h S c i e n c e s

N o r t h e a s t e r n I l l i n o i s U n i v e r s i t y C h i c a g o , I 1 l i n o i s 6 0 6 2 5

A1 U. F a l s t e r 9 2 0 M c I n t o s h S t r e e t

Wausau, W i s c o n s i n 5 4 4 0 1

* F o r s a l e b y P a u l E. M y e r s , D e p a r t m e n t o f G e o l o g y , U n i v e r s i t y

o f W i s c o n s i n , Eau C l a i r e , W i s c o n s i n 5 4 7 0 1 [ $ 6 . 0 0 ]

INTRODUCTION

This Guidebook is a major revision of one used in 1980 for the 26th AnnualInstitute on Lake Superior Geology. That volume was written by Sood, Myers,and Berlin. Subsequent more detailed mapping and analyses of rocks and mineralsfrom the southern portion of the Complex have necessitated this revision. A

comprehensive paper on the geology and petrology of the Wausau Syenite Complexis forthcoming.

Syenitic rocks of the Wausau area were first described by Weidman (1907) aspart of a general report on the geology of northern Wisconsin. A greater thannormal amount of attention was devoted in that report to the mineralogy of thesyenites. Emmons and Snyder (1944), Turner (1948), and Giesse (1951) subsequentlystudied mainly the mineral associations of the Wausau Syenite. The plutons of theWausau Syenite Complex were mapped in 1971—1976 as part of a mapping project bythe Wisconsin Geological and Natural History Survey (LaBerge and Myers, 1984).Koellner (1974) studied the mineral chemistry of the various mapped units in theStettin syenite pluton. Her excellent data have been incorporated in this guide-book with gratitude. Additional geochemical and mineral chemical-petrographicstudies are being carried on by Falster (1984), Myers, Medaris,(University ofWisconsin — Madison), and Sood (Northeastern Illinois University).

GENERAL GEOLOGY

Central Wisconsin is on the southernedge of the exposed Precambrian Shield(Figure 1). The Precambrian rocks are sparsely exposed through glacial cover,along streams, and as scattered inliers. The aeromagnetic map by Zietz and others(1978) and the Bouguer anomaly gravity map by Ervin and Hammer (1974) permitextrapolation of contacts between isolate exposures.

Archean migmatites, gneisses, and schists of Archean age are confined tolenticular fault slices in the region between Stevens Point and Black River Falls.They have been referred to informally (LaBerge and Myers, 1984, p. 247) as the"Stevens Point Complex," and U-Pb zircon age dated at more than 2,800 m.y. old.by Van Schmus (1977, 1980). Syenitic augen gneiss at Neillsville yields a U—Pbzircon age of 2,535 + 10 m.y. (Van Schmus, 1980). Archean gneisses and amphibolitesoccur north of the Niagara fault in northern Wisconsin, and it has been suggested(LaBerge, oral communication, 1984) that no Archeancrust underlies the regionbetween these roughly parallel east-northeast-trending fault systems.

Older Proterozoic rocks of the Wausau region are quartz—sillimanite schist,biotite schist, and amphibolite-grade metavolcanic rocks northwest of the Athensfault. Amphibolitic gneisses of volcanic and gabbroic parentage and tonaliticto trondhjemitic orthogneisses make up an east—northeast—trending belt of amphibo—lite—grade rocks comprising the Chippewa amphibolite complex (Myers,. l974)in theregion west and north of Marathon County. Amphibolitic rocks of similar lithologyare interleaved with Archean migmatites in the "Stevens Point complex" south ofMarathon County. The older succession is characterized by amphibolite-grade meta-morphic mineralogy and west- to northwest—plunging isoclinal folds deformed bymore open, coaxial folds. Basal quartzite and pelitic metasedimentary rocks inthis older succession are overlain by volcanic rocks, and intruded by numeroussyntectonic tonalites, trondhjemites, and granodiorites (LaBerge and Myers, 1984,p. 246).

The younger Early Proterozoic succession consists primarily of greenschist-faciescalc-alkaline basalt-rhyolite sequence that unconformably overlies the oldersuccession (Myers and others, 1980). Structures in the greenschist sequence

INTRODUCTION

This Guidebook i s a major revision of one used in 1980 for the 26th Annual Ins t i tu te on Lake Superior Geology. That volume was written by Sood, Myers, and Berlin. Subsequent more detailed mapping and analyses of rocks and minerals from the southern portion of the Complex have necessitated th i s revision. A comprehensive paper on the geology and petrology of the Wausau Syenite Complex i s forthcoming.

Syenitic rocks of the Wausau area were f i r s t described by Weidman (1907) as part of a general report on the geology of northern Wisconsin. A greater than normal amount of attention was devoted in that report t o the mineralogy of the syeni tes . Emmons and Snyder (1 944), Turner (1 948), and Giesse (1 951 ) subsequently studied mainly the mineral associations of the Wausau Syenite. The plutons of the Wausau Syenite Complex were mapped in 1971-1 976 as part of a mapping project by the Wisconsin Geological and Natural History Survey (LaBerge and Myers, 1984). Koellner (1 974) studied the mineral chemistry of the various mapped units in the S te t t in syenite pluton. Her excellent data have been incorporated in th i s guidebook with gratitude. Additional geochemical and mineral chemical-petrographic studies are being carried on by Falster (1984), Myers, Medaris,(University of Wisconsin - Madison), and Sood (Northeastern I1 1 inois University).

G E N E R A L G E O L O G Y

Central Wisconsin i s on the southernedge of the exposed Precambrian Shield ( ~ i g u r e 1 ) . The Precambrian rocks are sparsely exposed through glacial cover, along streams, and as scattered in l ie rs . The aeromagnetic map by Zietz and others (1 978) and the Bouguer anomaly gravity map by Ervin and Hammer (1 974) permit extrapolation of contacts between isolate exposures.

Archean migmatites, gneisses, and schis ts of Archean age are confined to lent icular fau l t s l ices in the region between Stevens Point and Black River Falls. They have been referred t o informally (LaBerge and Flyers, 1984, p. 247 ) as the "Stevens Point Complex," and U-Pb zircon age dated a t more than 2,800 m.y. old. by Van Schmus ( 1 9 7 7 , 1980). Syenitic augen gneiss a t Nei l lsvi l le yields a U-Pb zircon age of 2,535 + 10 m.y. (Van Schmus, 1980). Archean gneisses and amphibol i t e s occur north of the ~Tagara fau l t in northern Wisconsin, and i t has been suggested (LaBerge, oral communication, 1984) that no Archean crust underlies the region between these roughly para1 1 el east-northeast-trending f au l t systems.

Older Proterozoic rocks of the Wausau region are quartz-sillimanite schis t , b io t i te schis t , and amphibolite-grade metavolcanic rocks northwest of the Athens fau l t . Amphibolitic gneisses of volcanic and gabbroic parentage a n d t ona l i t i c t o trondhjemitic orthogneisses make u p an east-northeast-trending belt of amphibo- 1 i te-grade rocks comprising the Chippewa amphi bol i t e compl ex (Myers, 1974)in the region west and north of Marathon County. Amphibolitic rocks of similar lithology are interleaved with Archean migmatites in the "Stevens Point complex" south of Marathon County. The older succession i s characterized by amphibolite-grade metamorphic mineralogy a n d west- t o northwest-plunging isocl inal folds deformed by more open, coaxial folds. Basal quartzite and pe l i t i c metasedimentary rocks in th i s older succession are overlain by volcanic rocks, and intruded by numerous syntectonic tonal i t e s , trondh jemi tes , and granodiori tes (LaBerge and Myers, 1984, p. 246).

The younger Early Proterozoic succession consists primarily of greenschist-facies cal c-a1 kal ine basal t-rhyol i t e sequence that unconformably over1 ies the 01 der succession (Myers and others, 1980). Structures in the greenschist sequence

ofWEST-CENTRAL WISCONSIN*

EXPLANATIONRapakivi—type granites and associated intrusive rocks of the Wolf River batholith

Syenite, quartz syenite, quartz monzonite of the Wausau syenite complex

'—'---—-'-UNC ON F ORM ITY —--.-——-'....——

Metagabbro

Anorthosite

Calc—alkaline intrusive rocks consisting mainly of granite, granodiorite, tonalite,and quartz diorite; commonly foliated and/or lineated

Metasedimentary rocks including phyllite, chlorite schist, micaceous quartzite,tuffaceous sandstone, siltstone; greenschist facies.

Calc-alkaline metavolcanic rocks, mainly basaltic flows and andesite to rhyoliterocks; greenschist facies

—---U N CONFORM I T Y (?)—___—-—---

illllhJ!IJ Miphibolites

STRUCTURE SYMBOLS

'7 Approximate exposed edge of Precambrian basement

—7 Contact, dashed where inferred

L7 Contact based on aeromagnetic anomalies

Fault or shear zone, dashed where inferred; commonly wider than line

* From LaBerge and Myers, 1984.

—2—

FIGURE 1

GENERALIZED PRECAMBRIAN GEOLOGY

E: Quartzite

FIGURE 1

GENERALIZED PRECAMBRIAN GEOLOGY

o f - NEST-CENTRAL WISCONSIN*

E X P L A N A T I O N

Rapak iv i - type g r a n i t e s and assoc ia ted i n t r u s i v e rocks o f t h e Wolf R i v e r b a t h o l i t h

Syen i te , q u a r t z syen i t e , qua r t z monzonite o f t h e Wausau s y e n i t e complex

Q u a r t z i t e

- - - ' - - - U N C O N F O R M I T Y - = Metagabbro

A n o r t h o s i t e

C a l c - a l k a l i n e i n t r u s i v e rocks c o n s i s t i n g ma in l y o f g r a n i t e , g r a n o d i o r i t e , t o n a l i t e , and qua r t z d i o r i t e ; commonly f o l i a t e d and/or l i n e a t e d

Metasedimentary rocks i n c l u d i n g p h y l l i t e , c h l o r i t e s c h i s t , rnicaceous q u a r t z i t e , t u f f aceous sandstone, s i l t s t o n e ; g reensch i s t f a c i e s .

Calc-a1 k a l i n e metavo lcan ic rocks, m a i n l y b a s a l t i c f l o w s and andes i t e t o r h y o l i t e p y r o c l a s t i c rocks; g reensch i s t f a c i e s

- U N C O N F O R M I T Y (?)-

Lm h p h i b o l i t e s

STRUCTURE SYMBOLS

7 Approximate exposed edge o f Precambrian basement

---2 Contact, dashed where i n f e r r e d

CL> Contact based on aeromagnetic anomalies

R## F a u l t o r shear zone, dashed where i n f e r r e d ; c m o n l y w ider than l i n e

* From LaBerge and Myers, 1984.

-3—

trend northeasterly and plunge easterly. LaBerge and Myers (1984) proposed thatthe amphibolite—grade succession is pre—Penokean in age and that it representsa major, yet-unrecognized part of Wisconsin Precambrian history. The amphibolite-grade succession may correlate with the Chocolay Group in western Michigan andnorthern Wisconsin, while the greenschist rocks may correlate with the Menomonieand/or Baraga Groups. The isoclinal folds, higher grade metamorphism and tonaliticintrusives in the older succession may thus represent an older, pre-Penokeanorogeny.

The Early Proterozoic amphibolite-grade and greenschist—grade rocks are overlainnconformably by rhyolite pyroclastics and intruded by anorogenic granites ofapproximately 1760 n.y. age (Smith, 1978). Similar rocks in the Brokaw areanorth of Wausau may belong to this younger sequence. Possibly also associatedwith these volcanic and intrusive rocks are younger quartzites such as thoseexposed at Baraboo, Flambeaü Ridge, and Barron Hills, Wisconsin. The Rib Mountainand Mosinee Hill quartzites may also be of this age. These metasedimentary rocksare associated locally with dolomite, ferruginous slate, nietaconglomerate and chert.

Widespread folding and wrench faulting at between 1630-1600 n.y. ago was ofsufficient intensity to produce a significant resetting of Rb—Sr isotope systems.These wrench fault systems trend about east—west in the Stevens Point—Neillsvillearea and east—northeast in the region northwest of Wausau. The Jump River, Gilman,Monico, and Athens faults were probably developed or reactivated during this majorkinematic event.

Although the Wausau Syenite Complex (WSC) was intruded after most of the faultingin this region, some reactivation of old faults caused minor offsets and localcataclasi s..


The Wausau syenite complex (WSC) and coeval Wolf River batholith (WRB) are partof a NE—SW—trending belt of 1,770 to 1,030 n.y. anorogenic, granite plutons thatextends from the Baltic Shield to the southwestern United States (Figure 2).These plutons represent the last major episode of Precambrian granitic intrusionin the Lake Superior region. Anderson (1983) has compared the characteristics ofthese anorogenic plutons, and shows that the dominant type is of rapakivi affinity.According to Anderson (1983, p. 133) crystallization of these potassic, iron-enrichedmagmas took place at temperatures between 640 and 790°C and low total pressures,generally less than 2 kb. The present linear continuity of this belt of roughlycoeval plutons and their apparent shallow depth of intrusion signify a lack ofmajor subsequent continental rearrangement or major uplift. He concludes thatcrustal derivation of the magmas was a result of thermal doming in the mantle andthat cogenetic anorthositic and mangeritic magma emplacement may have played anactive role in generating necessary heat of fusion at lower crustal levels. Andersonfeels that the plutons do not show a consistent age progression of a track, andthat the thermal event was related to development of a failed rift.

Four concentrically—zoned, cylindrical alkaline plutons make up the WSC. Accord-ing to Van Schmus and others (l975a, l975b, 1981), The Stettin syenite plutoncrystallized 1,520 m.y. ago, while the WRB and Ninemile granite pluton crystallized1,500 n.y. ago. He feels that the U—Pb systems are clean enough to assure thatthe difference in ages is real. This difference in radiometric age is in accordwith cross-cutting relationships shown in the field. The oldest, most alkalineStettin pluton (Figure 3, #1) has a wall zone and core rimmed by nepheline syenite.Following the emplacement of the Stetting pluton, three pipe—like plutons wereintruded in series south-southwest from Wausau: the Wausau pluton (Figure 3, #2),the Rib Mountain pluton consisting mostly of quartz syenite with numerous largexenoliths (Figure 3, #3), and finally the Ninemile granite pluton (Figure 3, #4).

trend northeasterly and plunge easterly. LaBerge and Myers (1984) proposed that the amphibolite-grade succession i s pre-Penokean in age and that i t represents a majory yet-unrecognized part of Wisconsin Precambrian history. The amphibolite- grade succession may correlate with the Chocolay Group in western Michigan and northern Wisconsiny while the greenschist rocks may correlate with the Menomonie and/or Baraga Groups. The isocl inal fo1 ds higher grade metamorphism and tonal i t i c intrusives in the older succession may thus represent an oldery pre-Penokean orogeny.

The Early Proterozoic amphi bol i te-grade and greenschist-grade rocks are over1 ain nconformably by rhyolite pyroclastics and intruded by anorogenic granites of approximately 1760 may. age (Smithy 1978). Similar rocks in the Brokaw area north of Wausau may belong t o t h i s younger sequence. Possibly also associated with these volcanic and intrusive rocks are younger quartzites such as those exposed a t Barabooy F1 ambeau Ridgey and Barron Hi1 1 s Wisconsin. The Rib Mountain and Mosinee Hill quartzites may also be of t h i s age. These metasedimentary rocks are associated locally with dolomitey ferruginous s l a t e y metaconglomerate and chert.

Widespread folding and wrench faulting a t between 1630-1600 m.y. ago was of suff ic ient intensi ty t o produce a significant resett ing of Rb-Sr isotope systems. These wrench faul t systems trend about east-west in the Stevens Point-Neillsville area and east-northeast in the region northwest of Wausau. The Jump River* Gilmany Monicoy and Athens faul ts were probably developed or reactivated during th i s major kinematic event.

Although the Wausau Syenite Complex (WSC) was intruded a f t e r most of the faulting in th i s regiony some reactivation of old faul ts caused minor offsets and local catacl asis,.


The Wausau syenite complex (WSC) and coeval Wolf River bath01 i t h ( W R B ) are part of a NE-SW-trending bel t of 1 Â¶77 t o 1 Â¶O3 may. anorogenicy granite plutons that extends from the Ba1 t i c Shield to the southwestern United States (Figure 2 ) . These plutons represent the l a s t major episode of Precambrian grani t ic intrusion in the Lake Superior regiono Anderson (1983) has compared the character is t ics of these anorogenic plutonsy and shows that the dominant type i s of rapakivi a f f in i ty . According t o Anderson (1 983Â p. 133) crystal 1 ization of these potassicy iron-enriched magmas t o o k place a t temperatures between 640 and 790Â° and low total pressuresy generally less than 2 kb. The present l inear continuity of t h i s bel t of roughly coeval plutons and the i r apparent shallow depth of intrusion signify a lack of major subsequent continental rearrangement or major up1 i f t . He concl udes that crustal derivation of the magmas was a resul t of thermal doming in the mantle and that cogenetic anorthosit ic and mangeritic magma emplacement may have played an active role in generating necessary heat of fusion a t lower crustal levels. Anderson feels that the p1 utons do not show a consistent age progression of a t racky and that the thermal event was related to development of a fai led r i f t .

Four concentrically-zonedy cylindrical alkaline plutons make u p the WSC. Accord- ing to Van Schmus and others (1 975ay 1 975by 1981 ) Â The Ste t t in syenite p1 uton crystall ized 1 y520 m.y. agoy while the WRB and Ninemile granite pluton crystall ized 1,500 m.y. ago. He feels that the U-Pb systems are clean enough to assure that the difference in ages i s real. This difference in radiometric age i s in accord with cross-cutting relationships shown in the f ie ld. The o ldes ty most alkaline S te t t in pluton (Figure 3, # I ) has a wall zone and core rimmed by nepheline syenite. Fo1 lowing the emplacement of the Stet t ing p1 uton three pipe-1 i ke p1 utons were intruded in ser ies south-southwest from Wausau: the Wausau pluton (Figure #2) the Rib Mountain pluton consisting mostly of quartz syenite with numerous large xenoliths (Figure 3Â #3) , and f ina l ly the Ninemile granite pluton (Figure s y #4).

-4-

Figure 2 —- Proterozoic anorogenic granite com-plexes of North America (Anderson, 1983, p. 135)

Figure 3 —— Components of the Wausausyenite complex.

//

042miles

F i g u r e 2 -- P r o t e r o z o i c anorogenic g r a n i t e com- p l e x e s o f N o r t h America (Anderson, 1983, p. 135)

F i g u r e 3 -- Components o f t h e Wausau s y e n i t e compl ex.

—5-.

The southern half of the Wausau pluton was stoped by the Rib Mountain pluton, whosecore and southern rim were occupied by the Ninemile granite pluton. Granite andquartz monzonite aplites form a partial core rim in the southern lobe of the Nine.-mile pluton, and occupy several large, irregular areas west, southeast, and northof the Wasuau complex. Subhorizontal pegmatite dikes and pods in the Ninemilegranite contain miarolitic cavities whose mineral assemblages indicate thermalshock and shallow crystallization (Faister, 1984). Several younger, possiblyrelated quartz monzonite porphyry plugs were intruded across faults that cut thesyenites. The Wausau pluton has a small core of granite which closely resemblesthe Ninemile granite. In general the intrusive sequence represents a continualenrichment of the differentiating magmas in silica.

Figure 4 is a geologic map of Marathon County, which shows the context of theWausau syenite complex and its discordance with respect to east-northeasterlytrends in the older rocks. A portion of the Wolf River batholith can be seenin the eastern quarter of Marathon County. The Wolf River batholith is separatedfrom the Wausau syenite pluton by a major fault system which bends abruptly westwardalong the southern border of Marathon County. The Wausau syenite complex occupiesthe concave portion of this bend. It is possible that the syenite complex had animportant kinematic influence — as that of a knot - on the development of thefaults, although the syenite is clearly offset by many of them (later reactivation).

Figure 5 is a geologic map of the Wausau syenite complex showing the stops forthis field trip. Several extra stop descriptions are included so that one mayuse the field guide for self—guided field trips. The base map for the geologyis from LaBerge and Myers 1984 report on the Geology of Marathon County beingpublished by the Wisconsin Geological and Natural Survey. Additional mappingsince submission of the map for the Survey report has resulted in some revisions -particularly of the Wausau, Rib Mountain, and Ninemile plutons.

Figure 6 is a topographic map showing locations of stops and routes. Some ofthese will not be visited during the field trip owing to time limitations. Thesemaps were plotted on the following 15' topographic quadrangles (clockwise from thenorthwest: Hamburg, Merrill, Wausau, and Marathon.

The southern h a l f o f t h e Wausau p l u t o n was s toped by t h e R ib Mountain p l u ton , whose core and southern r i m were occupied by t h e N inemi le g r a n i t e p lu ton . G ran i t e and qua r t z monzoni te a p l i t e s form a p a r t i a l co re r i m i n t h e sou thern l o b e o f t h e Nine- m i l e p l u t o n 9 and occupy severa l l a r g e 9 i r r e g u l a r areas west, southeast , and n o r t h o f t h e Wasuau complex. Subhor i zon ta l pegmat i te d i k e s and pods i n t h e N inemi le g r a n i t e c o n t a i n m i a r o l i t i c c a v i t i e s whose m ine ra l assemblages i n d i c a t e thermal shock and sha l l ow c r y s t a l 1 i z a t i o n (Fa1s te r9 1984). Several younger, p o s s i b l y r e l a t e d qua r t z monzoni te porphyry p lugs were i n t r u d e d across f a u l t s t h a t c u t t h e syen i tes . The Wausau p l u t o n has a smal l co re o f g r a n i t e which c l o s e l y resembles t h e N inemi le g r a n i t e . I n genera1 t h e i n t r u s i v e sequence represen ts a c o n t i n u a l enr ichment o f t h e d i f f e r e n t i a t i n g magmas i n s i l i c a .

F i gu re 4 i s a geo log i c map o f Marathon County, which shows t h e c o n t e x t o f t h e Wausau s y e n i t e complex and i t s d iscordance w i t h r espec t t o e a s t - n o r t h e a s t e r l y t r ends i n t h e o l d e r rocks. A p o r t i o n o f t h e Wol f R i v e r b a t h o l i t h can be seen i n t h e eas te rn q u a r t e r o f Marathon County. The Wol f R i v e r b a t h o l i t h i s separated from t h e Wausau s y e n i t e p l u t o n by a ma jo r f a u l t system which bends a b r u p t l y westward a l ong t h e southern border o f Marathon County. The Wausau s y e n i t e complex occupies t h e concave p o r t i o n o f t h i s bend. It i s p o s s i b l e t h a t t h e s y e n i t e complex had an impo r tan t k i nema t i c i n f l u e n c e - as t h a t o f a kno t - on t h e development o f t h e f a u l t s , a1 though t h e s y e n i t e i s c l e a r l y o f f s e t by many o f them ( l a t e r r e a c t i v a t i o n ) .

F i gu re 5 i s a geo log i c map o f t h e Wausau s y e n i t e complex showing t h e s tops f o r t h i s f i e l d t r i p . Several e x t r a s t o p d e s c r i p t i o n s a re i n c l u d e d so t h a t one may use t h e f i e l d gu ide f o r s e l f - g u i d e d f i e l d t r i p s . The base map f o r t h e geology i s f rom LaBerge and Myers 1984 r e p o r t on t h e Geology o f Marathon County be ing pub1 i shed by t h e Wisconsin Geolog ica l and Na tu ra l Survey. A d d i t i o n a l mapping s i n c e submission o f t h e map f o r t h e Survey r e p o r t has r e s u l t e d i n some r e v i s i o n s - p a r t i c u l a r l y o f t h e Wausau, R i b Mountain, and N inemi le p1 utons.

F i gu re 6 i s a topograph ic map showing l o c a t i o n s o f s tops and rou tes . Some o f these w i l l n o t be v i s i t e d d u r i n g t h e f i e l d t r i p owing t o t ime l i m i t a t i o n s . These maps were p l o t t e d on t h e f o l l o w i n g 1 5 ' topograph ic quadrangles ( c l ockw i se f rom t h e nor thwes t : Hamburg, M e r r i l l , Wausau, and Marathon.

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EXPLANATION

AlluviumGlacial outwashGlacial tillUNCONFORMITY—.._---—...

db Diabase Dike

qp Quartz monzonite porphyryga Granite apliteng Ninemile granite and

quartz monzoniteqs Quartz syeniteas Amphibole syenitePS Pyroxene—bearing syenite

sv Syenitized volcanic rockssa Syenite aplite

(Border phase)is Hybrid lensoidal syenitens Nepheline syenitets Tabular syenite

ig Leucocratic graniteqm Quartz monzonite

qd Quartz diorite and diorite

vs Volcanogenic sedimentaryrocks

fv Felsic volcanic rocksiv Intermediate volcanic rocksmy Mafic volcanic rocks

q Metaquartzitebs Biotite schistam Amphibolite

Contact: dashed wheredashed where inferred; dot-

ted where covered

Fault: dashed whereinferred; dotted where

covered70

Strike and dip of layering80

Stike and dip of foliation

GEOLOGIC MAP OF THE

WAUSAU SYENITE COMPLEX,

CENTRAL WISCONSINy Paul E. Myers

LATEPROTERO-zol C

STRUCTURE SYMBOLS

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LIScale, miles

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1984

Figure 5 ——

EXPLANATION f a Al luvium e-i

STRUCTURE SYMBOLS -

go Glac ia l outwash N /-l.-#..-l

g t Glac ia l t i l l UJ t_> - UNCONFORMITY- Contact: dashed where

dashed where in fe r red ; dot- LATE ted where covered

PROTERO- db Diabase Dike ZOIC

A QD Quartz monzonite ~ o r o h v r v I '' .

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Granite apl i t e in fe r red ; dot ted where Ninemile g ran i te and quartz monzoni t e

Quartz syeni te

Amphibole syeni te

Pyroxene-bearing syeni t e

Syeni t i z e d vo lcanic rocks

Syeni t e apl i t e (Border phase)

Hybrid lensoidal syeni te

Nephel i n e syeni te

Tabular syeni te

Leucocrat ic g ran i te

Quartz monzonite

Quartz d i o r i t e and d i o r i t e

Vol canogenic sedimentary rocks

Fe ls i c vo lcanic rocks

Intermediate volcanic rocks

Mafic volcanic rocks

Metaquartzi t e

B i o t i t e s c h i s t

Amphi bol i t e

covered 70 \

S t r i k e and d i p o f l a y e r i n g so <

Stike and d i p o f f o l i a t i o n

MAP AREA /'""-I Scale, mi les

1984

Figure 5 --

GEOLOGIC MAP OF THE

WAUSAU SYENITE COMPLEX,

CENTRAL WISCONSIN by Paul E. Myers

—7-

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-9—

STOP #1

TITLE: RIB MOUNTAIN SUMMIT - GENERAL GEOLOGY

LOCATION: Rib Mountain State Park observation platform: SE/4 Sec. 8, T28N,R7E; Wausau 15' Quadrangle, Wausau West 7.5' Quadrangle

DATE: March, 1984

DESCRIPTION:

From this vantage point, one can get a general perspective of the majorelements of the geology and geography of the Wausau region. This observationtower and all of Rib Mountain are on the upturned edge of nearly vertical bedsof very coarse—grained metaquartzite with tops facing southward. Facing directionis indicated by sparsely exposed cross—bedding and occasional ripple forms.The Rib Mountain quartzite is an arcuate, keel-shaped xenolith embedded inquartz syenite: it is situated on the northern edge of the Rib Mountainsyenite-quartz syenite pluton, whose core (just south of here) is occupied b'granite and quartz monzonite of the northern lobe of the Ninemile pluton.Mosinee Hill southeast of here (the knob with the radio antenna on it) andHardwood hill southwest of here are similar, but smaller quartzite xenolithremanents that are located in analogous positions in the cylindrical RibMountain pluton. The quartzite xenoliths form about three—quarters of a circlewith a diameter of about 5 miles. Relict bedding in the quartzites at MosineeHill (STOP #2) and Hardwood Hill is parallel to elongation direction of thelenticular xenoliths.

A large, abandoned quarry, operated by 3M Corporation as a source of roofinggranules, affords an excellent exposure of the quartzites. A porphyritic diabasedike cuts through quartzites on the south wall of the quarry, and is traceablein float for about 2 miles to the west—southwest. The quarry is located on thewest end of Rib Mountain about one-half mile west of here.

The east end of the Rib Mountain quartzite body is offset by a small northeast-trending fault. This fault is parallel with larger faults of similar trend thatcut the edges of the Rib Mountain pluton.

The Wausau pluton and the Rib Mountain pluton show an uncanny resemblance: theircontact is covered by Rib River alluvium just north of Rib Mountain. Each ofthe plutons is cored by granite, and each contains a large mass of metaquartzitein its northern edge. One is tempted to suggest that the apparent "duplication"was produced by low-angle faulting. However, the two pl'itons are significantlydifferent in mineral composition and in the types of xenoliths making up theirintermediate zones. The Wausau syenite has a broad intermediate zone of amphibolesyenite containing mainly metavolcanic xenoliths, while the Rib Mountain plutonintermediate zone is composed dominantly of quartz syenite (with a texture stronglyresembling arkose) and xenoliths mainly of metaquartzite, biotite schist, andamphibol ite.

Interesting questions arise as to the origin and mode of emplacement of thesexenoliths. The schists, amphibolite, and possibly at least some of the meta-quartzite xenoliths show higher grades of regional metamorphism than is displayedin the rocks surrounding the Wausau complex at its present level of exposure.It is therefore inferred that these xenoliths were brought up from a more highlymetamorphosed basement. Volcanic xenoliths, like those seen in the Wausau syeniteat Stop 3 may represent material collapsed into the pluton during caldera subsidence.

STOP #1

TITLE- - RIB MOUNTAIN SUMMIT - GENERAL GEOLOGY

LOCATION: R i b Mountain S t a t e Park obse rva t i on p l a t f o r m : SE/4 Sec. 8, T28N, R7E; Wausau 15 ' Quadrangle, Wausau West 7.5' Quadrangle

DATE : - March, 1984

DESCRIPTION:

From t h i s vantage p o i n t , one can ge t a general pe r spec t i ve o f t h e major elements o f t h e geology and geography o f t h e Wausau reg ion . Th i s obse rva t i on tower and a l l o f R ib Mountain a r e on t h e upturned edge o f n e a r l y v e r t i c a l beds o f ve r y coarse-gra ined m e t a q u a r t z i t e w i t h tops f a c i n g southward. Fac ing d i r e c t i o n i s i n d i c a t e d by spa rse l y exposed cross-bedding and occas iona l r i p p l e forms. The R ib Mountain q u a r t z i t e i s an arcuate, keel-shaped x e n o l i t h embedded i n qua r t z syen i t e : i t i s s i t u a t e d on t h e n o r t h e r n edge o f t h e R ib Mountain syen i t e -qua r t z s y e n i t e p l u ton , whose core ( j u s t sou th o f he re ) i s occupied by g r a n i t e and qua r t z monzoni te o f t h e n o r t h e r n l obe o f t h e N inemi le p l u ton . Mosinee H i1 1 sou theas t o f here ( t h e knob w i t h t h e r a d i o antenna on i t ) and Hardwood h i l l southwest o f here a r e s i m i l a r , b u t sma l l e r q u a r t z i t e x e n o l i t h remanents t h a t a re l o c a t e d i n analogous p o s i t i o n s i n t h e c y l i n d r i c a l R i b Mountain p lu ton . The q u a r t z i t e x e n o l i t h s form about t h ree -qua r t e r s o f a c i r c l e w i t h a d iameter o f about 5 m i l es . R e l i c t bedding i n t h e q u a r t z i t e s a t Mosinee H i l l (STOP #2) and Hardwood H i l l i s p a r a l l e l t o e l o n g a t i o n d i r e c t i o n o f t h e l e n t i c u l a r xenol i ths .

A l a rge , abandoned quar ry , operated by 3M Co rpo ra t i on as a source o f r o o f i n g granules, a f f o r d s an e x c e l l e n t exposure o f t h e q u a r t z i t e s . A p o r p h y r i t i c d iabase d i k e c u t s th rough q u a r t z i t e s on t h e sou th w a l l o f t h e quar ry , and i s t r a c e a b l e i n f l o a t f o r about 2 m i l e s t o t h e west-southwest. The qua r r y i s l o c a t e d on t h e west end o f R ib Mounta in about one -ha l f m i l e west o f here.

The eas t end o f t h e R ib Mountain q u a r t z i t e body i s o f f s e t by a smal l no r t heas t - t r e n d i n g f a u l t . Th i s f a u l t i s p a r a l l e l w i t h l a r g e r f a u l t s o f s i m i l a r t r e n d t h a t c u t t h e edges o f t h e R ib Mountain p l u ton .

The Wausau p l u t o n and t h e R ib Mountain p l u t o n show an uncanny resemblance: t h e i r c o n t a c t i s covered by R ib R i v e r a l l u v i u m j u s t n o r t h o f R ib Mountain. Each o f t h e p l u tons i s cored b y g r a n i t e , and each con ta i ns a l a r g e mass o f m e t a q u a r t z i t e i n i t s n o r t h e r n edge. One i s tempted t o suggest t h a t t h e apparent " d u p l i c a t i o n " was produced by 1 ow-angl e f a u l t i n g . However, t h e two p i u tons a re s i gn i f i cant1 y d i f f e r e n t i n m ine ra l compos i t i on and i n t h e t ypes o f x e n o l i t h s making up t h e i r i n t e r m e d i a t e zones. The Wausau s y e n i t e has a broad i n t e r m e d i a t e zone o f amphibole s y e n i t e c o n t a i n i n g m a i n l y metavo lcan ic x e n o l i t h s , w h i l e t h e R i b Mounta in p l u t o n i n t e r m e d i a t e zone i s composed dominan t l y o f qua r t z s y e n i t e ( w i t h a t e x t u r e s t r o n g l y resembl i n g a rkose) and xenol i t h s m a i n l y o f me taqua r t z i t e , b i o t i t e s c h i s t , and amphi bo l i t e .

I n t e r e s t i n g ques t ions a r i s e as t o t h e o r i g i n and mode o f emplacement o f these x e n o l i t h s . The s c h i s t s , amph ibo l i te , and p o s s i b l y a t l e a s t some o f t h e meta- q u a r t z i t e x e n o l i t h s show h i g h e r grades o f r e g i o n a l metamorphism than i s d i s p l a y e d i n t h e rocks su r round ing t h e Wausau complex a t i t s p resen t l e v e l o f exposure. I t i s t h e r e f o r e i n f e r r e d t h a t these x e n o l i t h s were b rough t up f rom a more h i g h l y metamorphosed basement. Vo lcan ic x e n o l i t h s , l i k e those seen i n t h e Wausau s y e n i t e a t Stop 3 may rep resen t m a t e r i a l co l l apsed i n t o t h e p l u t o n d u r i n g ca lde ra subsidence.

TITLE: Large Quartzite and Biotite Schist Xenoliths in the CoreRim, Wausau Syenite Pluton

LOCATION: South end of Mosinee Hill, NE¼, NE¼ Sec.27, T28N, R7EWausau 15' and Wausau West 7.5' quadrangles

AUTHOR: Paul E. Myers, University of Wisconsin-Eau Claire

DATE: February, 1980

SUMMARY OF FEATURES:

This abandoned 3-M quarry exposes the south end of a large quartz-ite xenolith and a much smaller xenolith of biotite schist (Figure 1).The lensoidal shape of the large xenoliths is extrapolated from shapesof smaller ones throughout the intermediate zone. Near its contact withquartz syenite the quartzite is impregnated with very fine-grained, in-terstitial pink microcline which selectively replaced certain layersin the quartzite. The abundance of interstitial K-feldspar diminishestoward the center of the quartzite xenolith. Smaller quartzite xeno-liths have been thoroughly granitized. The question of whether thesexenoliths were carried up or down along the cylindrical wall of theWausau syenite pluton is still not answered.

The only significant bedrock occurrence of quartzite and biotiteschist in this area is as xenoliths in the Wausau syenite pluton. Thexenoliths have the following important characteristics:

1. They show concentric, zonal distribution and orientation aroundthe quartz monzonitic core—-the Ninemile pluton.

-10-

STOP # 2STOP # 2

TITLE: Large Quartzi te and B io t i t e Schis t Xenoliths in the Core R i m , Wausau Syeni t e Pl uton

LOCATION : South end of Mosinee Hill , NEky NEk Sec.27, TZ8N, R7E Wausau 15' and Wausau West 7.5' quadrangles

AUTHOR: Paul E . Myers, University of Wisconsin-Eau Cla i re

DATE : February , 1980


This abandoned 3-M quarry exposes the south end of a large quartz- i t e xenolith and a much smaller xenolith of b i o t i t e s c h i s t (Figure 1 ) . The lensoidal shape of the l a rge xenoli ths i s extrapolated from shapes of smaller ones throughout the intermediate zone. Near i t s contact w i t h quartz syeni te the quar tz i t e i s impregnated w i t h very fine-grained, i n - t e r s t i t i a l pink microcl ine which se lec t ive ly replaced ce r t a in layers i n the quar tz i t e . The abundance of i n t e r s t i t i a l K-feldspar diminishes toward the center of the quar tz i t e xenoli th. Smaller qua r t z i t e xeno- l i t h s have been thoroughly grani t ized. The question of whether these xenoli ths were carr ied up o r down along the cyl indr ical wall of t he Wausau syeni te pluton i s s t i l l not answered.

The only s ign i f ican t bedrock occurrence of qua r t z i t e and b i o t i t e s ch i s t in t h i s area i s as xenoli ths i n the Wausau syeni te pluton. The xenoliths have the following important charac te r i s t i cs :

1 . They show concentric, zonal d i s t r ibu t ion and or ienta t ion around the quartz monzonitic c o r e ~ t h e Ninemile pluton.

—11—

2. The largest xenoliths occur one mile outside the core.

3. The quartzite xenoliths are the largest because of their lowersusceptibility to fragmentation and assimilation.

4. Flow structure in quartz syenite and feldspar lenticulation in-dicate intrusion of the quartz syenite as a viscous crystal mush.

5. Mafic xenoliths were biotitized, and quartzite xenoliths weregranitized through the metasomatic addition of K 0 and Al 0with selective replacement of quartzite by fine-rained mco-dine along bedding planes.

6. Xenoliths north of the Rib River are dominantly metavolcanic rocks,whereas the xenoliths south of Rib River are dominantly quartzite,biotite schist and very subordinate non-foliated metadiabase.

7. Quartz grains in the quartz syenite and the outer part of theNinemile pluton are granular, subangular, coarse grained andstrained.

THE NINEMILE PLUTON:

The Ninemile pluton has a granite rim containing xenocrystic quartz.Samples taken at one-mile intervals across the pluton from north tosouth and from west to east show a decreasing percentage of xenocrysticquartz and an increasing amount of plagioclase toward the center of thepluton. The contact at the Ninemile pluton is locally discordant, as atBlack Creek 1.7 miles northwest of here. Miarolitic cavities, some filledwith large quartz crystals are common along the west side of the Ninemilepluton. They indicate shallow conditions of crystallization.

FIGURE 7-- Profile of the south end of Mosinee HillFIGURE 7-- Profile of the south end of Mosinee Hill

The largest xenoliths occur one mile outside the core.

The quartzi te xenoliths are the largest because of the i r lower suscept ibi l i ty to fragmentation and assimilation.

Flow structure in quartz syeni t e and feldspar lenticulation indicate intrusion of the quartz syenite as a viscous crystal mush.

Mafic xenol i ths were bioti t i zed, and quartzi te xenol i ths were granitized through the metasomatic addition of K 0 and A1 0 with selective rep1 acement of quartzite by f ine-grained m?c?o- cl i ne a1 ong bedding planes . Xenoliths north of the Rib River are dominantly metavolcanic rocks, whereas the xenoliths south of Rib River are dominantly quartzi te , b io t i t e schis t and very subordinate non-foliated metadiabase.

Quartz grains i n the quartz syenite and the outer part of the Ninemi l e pl uton are granular, subangular, coarse grained and strained.

THE NINEMILE PLUTON:

The Ninemile pluton has a granite rim containing xenocrystic quartz. Samples taken a t one-mile intervals across the pluton from north to south and from west to eas t show a decreasing percentage of xenocrystic quartz and an increasing amount of plagioclase toward the center of the pluton. The contact a t the Ninemile pluton i s locally discordant, as a t Black Creek 1.7 miles northwest of here. Miarol i t i c cavi t ies , some f i l l e d w i t h large quartz crystals are common along the west s ide of the Ninemile pl uton. They indicate shallow conditions of crystal 1 iza t i on.

—12-

Figure 8-Block diagram of the northeastern corner of the southernsegment of the Wausau syenite pluton at Mosinee Hill showing abundant,well—oriented quartzite (q) and biotite schist Cbs) xenoliths in a flow-laminated, lensoidal quartz syenite (lqsy). The Ninemile quartz monz-onite pluton (qm) intruded the quartz syenite with only a local discord-ance. The lensoidal syenite is bounded on the east by a thin wall ofamphibole syenite (asy) which is itself in fault contact eastward withfelsic volcanics. These rocks are cut with sharp discordance by aprominent diabase (db) dike which is characterized by a strong reversepolarity. The Qal is Wisconsin River alluvium. The shaded rectangleshows the icoation of the profile in Figure 7.

Figure 9—-— Camera lucida drawing of slabbed specimen of quartz syenite brecciacontaining aligned, angular clasts of banded quartzite (q), biotite schist (bs),and feldspar (f). Quartzite clast boundaries are typically sutured and embayedby K—feldspar metacrysts. This rock is interpreted as a caldera rim collapsebreccia. Specimen from the ridge west of Rib Flountain.

Figure 8--Block diagram o f the no r theas te rn corner of the southern segment of the Wausau syen i te p l u t o n a t Mosinee H i l l showing abundant, we l l -o r ien ted q u ? r t z i t e (q) and b i o t i t e s c h i s t (bs) x e n o l i t h s i n a f low- laminated, l enso ida l quar tz syeni t e ( lqsy) . The Ninemi le quar tz monz- o n i t e p l u t o n (am) In t ruded the quar tz syen i te w i t h o n l y a l o c a l d iscordance. The lenso ida l s e n i t e i s bounded on the eas t by a t h i n w a l l o f amphibole syen i te (asy) which i s i t s e l f i n f a u l t con tac t eastward w i t h f e l s i c vo lcanics. These rocks are c u t w i t h sharp discordance by a prominent diabase (db) d i k e which i s charac te r i zed by a s t rong reverse p o l a r i t y . The Qal i s Wisconsin R ive r a l luv ium. The shaded rec tang le shows the l c o a t i o n of the p r o f i l e i n F igu re 7 .

Fiqure 9-- Camera luc ida drawing o f slabbed specimen o f quar tz syen i te brecc ia con ta in ing al igned, angular c l a s t s o f banded q u a r t z i t e (q ) , b i o t i t e s c h i s t (bs) , and fe ldspar ( f ) . Q u a r t z i t e c l a s t boundaries are t y p i c a l l y sutured and embayed by K-feldspar metacrysts. This rock i s i n te rp re ted as a caldera r i m col lapse brecc ia. Specimen from the r i d g e west o f Rib Mountain.

Figure 10-- GranitizedMosinee Hill. Banding

quartzite from the east side ofis relict bedding.

Figure 11-.— Coarse microcline and quartz xenocrystsquartz syenite from the west side of Mosinee Hill.quartz grains have strain laniellae. The matrix isquartz and K—feldspar. Width of picture is 3.4 mm.

—13—

fromThe

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Figure lo-- Granitized quartzite from the east side of Mosinee Hill. Banding i s r e l i c t bedding.

Figure 11-- Coarse microcline and quartz xenocrysts from quartz syenite from the west side of Mosinee Hill . The quartz grains have s t ra in lamellae. The matrix i s angular quartz and K-feldspar. Width of picture i s 3 . 4 mm.

-14-

STOP #3

TITLE: Mineralogy of Pegmatite Bodies in the Ninemile PlutonLOCATION: North Central Sec0 19, 1 28 N, R 7 E, Wausau 15t Quadrangle. South side

of County Highway N about 4 miles west of US-51,[See map, page 8 1

AUTHORS: Al Faister, 920 McIntosh St., Wausau, and Paul Myers, University ofWisconsin — Eau Claire

DATE: March, 1984


The Ninemile pluton was emplaced at shallow depth (probably less than 4 km)in the core and southern rim of the Rib Mountain pluton and in the older meta-volcanic, metasedimentary, and plutonic rocks surrounding it. The Ninemilepluton is composed of biotite-hornblende granite and quartz monzonite, and islocally crowded with inclusions of quartzite, schist, and syenite. Pegmatite podsin the Ninemile pluton occur as: (1) small schlierenlike masses, (2) zoned dikeswith miarolitic cavities, (3) simple, poorly zoned bodies with vugs in the inter-mediate zone, and (4) late stage bodies showing selective etching and crystallizationof accessory minerals.

DESCRIPTION:

The Ninemile pluton is composed of K—feldspar, Na—feldspar (together usually,but not always as perthite), quartz, biotite and amphibole. Quartz grains haverounded—polygonal shape, and closely resemble grains from quartzite like thatexposed at Rib Mountain and Mosinee Hill . Hematite and Ti—rich biotite arelocally abundant. The Ninemile pluton contains numerous pegmatitic pods and dikes,which are shallow dipping except near the margin of the pluton, a factor suggestingpegmatite emplacement in concentric zones of thermal contraction. Deep weatheringof the Ninemile granites, especially in a circular zone near its rim, has produceda ferruginous gruss which has been quarried for use as paving materials for over90 years. The abundance of fluorite and the concentric structure of the Ninemilepluton suggest that the zones of most intense weathering reflect the local effectsof the formation of HF to cause etching and solution of quartz grains with consequentdisaggregation of the rock.

Pegmatite Types and Mineralogy

Study of pegmatites in the Ninemile pluton is hampered by poor exposure.However, more than 800 miarolitic cavities in over 800 pegmatite bodies have beenexamined and described in detail by A. Falster. The search for pegmatite bodiesis aided considerably by studying distribution of certain plants, infrared aerialphotography, and subtle variations in topography. The average size •of pockets isabout 15 centimeters in maximum dimension, although pockets up to 4.5 x 1.2 x 1 metershave been found. In addition to regular pockets, some pegmatite dikes containvuggy zones. Some very fine microcrystalline material has been taken from thesevugs.

1. Simple, schlieren-like masses of small size and simple mineralogy. These massesare common in the vicinity of larger dikes. Zoning is generally not well developed.Grain size increases regularly inward, with the coarsest crystals in the hangingwall near the center (indicating vapor nourishment). Mineralogy: microcline, albite,quartz, and a few accessories including siderite (pseudomorphed by goethite, hematite,and other minerals), hematite, hisingerite, second generation feldspar and quartz, andrare phenakite, anatase, sulfides, and sulfosalts.

STOP #3

TITLE : M i n e r a l o g y o f Pegmat i te Bod ies i n t h e N i n e m i l e P l u t o n

LOCATION: N o r t h C e n t r a l Seco 19, T 28 N, R 7 E, Wausau 1 5 ' Quadrangle. South s i d e o f County Highway N about 4 m i l e s west o f US-51 .[See map, page 8 ]

AUTHORS: A1 F a l s t e r , 920 Mc In tosh St., Wausau, and Paul Myers, U n i v e r s i t y o f Wiscons in - Eau C l a i r e

DATE : - March, 1984


The N i n e m i l e p l u t o n was emplaced a t s h a l l o w dep th ( p r o b a b l y l e s s t h a n 4 km) i n t h e c o r e and s o u t h e r n r i m o f t h e R i b Mounta in p l u t o n and i n t h e o l d e r meta- v o l c a n i c , metasedimentary, and p l u t o n i c r o c k s s u r r o u n d i n g i t . The N i n e m i l e p l u t o n i s composed o f b i o t i t e - h o r n b l e n d e g r a n i t e and q u a r t z monzoni te, and i s l o c a l l y crowded w i t h i n c l u s i o n s o f q u a r t z i t e , s c h i s t , and s y e n i t e . Pegmat i te pods i n t h e N i n e m i l e p l u t o n o c c u r as : (1 ) sma l l s c h l i e r e n l i k e masses, (2 ) zoned d i k e s w i t h m i a r o l i t i c c a v i t i e s , ( 3 ) s imp le , p o o r l y zoned bod ies w i t h vugs i n t h e i n t e r - media te zone, and ( 4 ) l a t e s t a g e bod ies showing s e l e c t i v e e t c h i n g and c r y s t a l l i z a t i o n o f accessory m i n e r a l s.

DESCRIPTION:

The N inemi l e p l u t o n i s composed o f K - f e l d s p a r , Na- fe l dspar ( t o g e t h e r u s u a l l y , b u t n o t a lways as p e r t h i t e ) , q u a r t z , b i o t i t e and amphibole. Q u a r t z g r a i n s have rounded-po lygonal shape, and c l o s e l y resemble g r a i n s f rom q u a r t z i t e 1 i ke t h a t exposed a t R i b Mounta in and Mosinee H i l l . Hemat i te and T i - r i c h b i o t i t e a r e l o c a l l y abundant. The N i n e m i l e p l u t o n c o n t a i n s numerous p e g m a t i t i c pods and d i k e s , wh ich a r e s h a l l o w d i p p i n g excep t near t h e marg in o f t h e p l u t o n , a f a c t o r s u g g e s t i n g pegmat i te emplacement i n c o n c e n t r i c zones o f thermal c o n t r a c t i o n . Deep w e a t h e r i n g o f t h e N i n e m i l e g r a n i t e s , e s p e c i a l l y i n a c i r c u l a r zone n e a r i t s r im , has produced a f e r r u g i n o u s gruss wh ich has been q u a r r i e d f o r use as p a v i n g m a t e r i a l s f o r o v e r 90 years. The abundance o f f l u o r i t e and t h e c o n c e n t r i c s t r u c t u r e o f t h e N i n e m i l e p l u i o n suggest t h a t t h e zones o o f t h e f o r m a t i o n o f HF t o cause d i s a g g r e g a t i o n o f t h e rock .

Pegmat i te Types and M i n e r a l o g y

S tudy o f pegmat i tes i n t h e However, more t h a n 800 m i a r o l i t

most i n t e n s e w e a t h e r i n g r e f l e c t t h e l o c a l e f f e c t s e t c h i n g and s o l u t i o n o f q u a r t z g r a i n s w i t h consequent

N inemi l e p l u t o n i s hampered b y poor exposure. i c c a v i t i e s i n o v e r 800 pegmat i te bod ies have been

examined and d e s c r i b e d i n d e t a i l b y A. F a l s t e r . The search f o r p e g m a t i t e bod ies i s a i d e d c o n s i d e r a b l y b y s t u d y i n g d i s t r i b u t i o n o f c e r t a i n p l a n t s , i n f r a r e d a e r i a l photography, and s u b t l e v a r i a t i o n s i n topography. The average s i z e o f pocke ts i s about 15 c e n t i m e t e r s i n maximum dimension, a l t h o u g h pocke ts up t o 4.5 x 1.2 x 1 meters have been found. I n a d d i t i o n t o r e g u l a r pocke ts , some pegmat i te d i k e s c o n t a i n vuggy zones. Some v e r y f i n e m i c r o c r y s t a l l i n e m a t e r i a l has been taken f r o m t h e s e vugs.

1. Simple, s c h l i e r e n - l i k e masses o f sma l l s i z e and s i m p l e m ine ra logy . These masses a r e common i n t h e v i c i n i t y o f l a r q e r d i k e s . Zon ing i s g e n e r a l l y n o t w e l l developed. G r a i n s i z e i n c r e a s e s r e g u l a r l y inward, w i t h t h e c o a r s e s t c r y s t a l s i n t h e hang ing w a l l n e a r t h e c e n t e r ( i n d i c a t i n g vapor nour i shment ) . M i n e r a l o g y : m i c r o c l i n e , a1 b i t e , q u a r t z , and a few a c c e s s o r i e s i n c l u d i n g s i d e r i t e (pseudomorphed b y g o e t h i t e , hemat i te , and o t h e r m i n e r a l s ) , hemat i te , h i s i n g e r i t e , second g e n e r a t i o n f e l d s p a r and q u a r t z , and r a r e phenak i te , anatase, s u l f i d e s , and s u l f o s a l t s .

—15—

2. Complexly zoned, typically large dikes with well-formed miarolitic cavities.Dikes of this kind are typically larger than 10 x 10 x 0.2 meters and exhibit well-defined zoning with wall zones, intermediate zones, and core zones. While thewall zones are usually poorly defined, the intermediate zones generally consistof graphic zones, blocky zones (usually near cores) and pocket zones of aplite.Mineralogy is diverse: microcline, albite, and quartz are the essential minerals,often occurring in two or more successive generations. Accessory minerals includeseveral or numerous species of the following: sulfides — pyrite, sphalerite, galena;sulfosalts — jamesonite, boulangeite and other identified species; carbonates —

siderite (commonly replaced by Fe' minerals like goethite, hematite, and lepido-crocite) calcite, mangano—calcite and ankerite(?); Be—minerals such as phenakite,bertrandite, bavenite, euclase, and beryl; and REE—rich (Th-poor) cheralite,xenotime, monazite, and synchisite—parisite.

Dikes of this type commonly show the effects of thermal shock: i.e. brecciatedpockets showing secondary overgrowths and cementation. In a few cases the pocketswere rapidly evacuated of all fluids, and metastable feldspar assemblages crystall-ized from escaping fluids while coating all earlier pocket phases.

3. Simple bodies with extensive vuggy regions in the intermediate zone.These dikes are large, thick, and do not show well—developed zoning. Wall and inter-mediate zones contain patchy quartz cores. Large miarolitic cavities are rare,but myriads of tiny vesicles abound in the intermediate zone. Maximum size ofthese cavities is about 0.5 - 1.5 centimeters. Dominant minerals are microcline,albite, quartz, and tiny (<1.0 millimeter) hematite crystals. Accessory mineralsinclude zircon, fluorite, F—apatite. Late stage milky quartz veins cut thesedikes in about 50 percent of the cases.

4. Pegmatite bodies with late stage selective etching and crystallization ofaccessory minerals. Although similar in many respects to the complexly zoned largedikes described above, they differ in several significant respects. They typicallyshow an inward diminution in quartz content owing to removal by corrosive fluids.Spaces originally filled with quartz were filled by graphic intergrowths of quartzand feldspar. Secondary growth of feldspars was accompanied by crystallization ofTi oxides (anatase brookite rutile) and ilmenite. Other minerals include Tibearing hematite (Ti = 2-3 percent), zircon, muscovite, bertrandite, phenakite,cheralite, xenotime, and others.

Internal Evolution of Megmatite Dikes (Types 2 and 4 above)Development of a very thin, fine-grained contact zone (0.5 - 2 centimeters)

was followed by crystallization of an extensive wall zone containing elongate biotite.Vapor nourishment fed crystals growing down from hangingwall. Finer grained partsof the wall zone grew upwards from the footwall, with occasional development ofa coarse-grained aggregate in pockets of vapor nourishment. Rising vapors initiallyformed coarse K—feldspar in graphic intergrowth with quartz, and later blockkyK—feldspar. Quartz formed large monomineralic masses in the upper—median part ofthe dike. A few very large crystals of siderophyllite formed at the core margin.As the pocket stage was approched a pressure quench may have affected the fluidyielding an aplite, while in other sections of the dike, miarolitic cavities formed.These cavities may be singular, relatively large openings, or they may be extensiveareas in the intermediate zone of tiny vesicles with material resembling a sponge.At later stages, metasomatism or hydrothermal replacement of some pocket consituentssets inwith alteration of siderite, pyrite, and other minerals to goethite, hematite,hydration of phenakite to bertrandite, corrosion of quartz and feldspars (adularia—habit), pocket rupture and thermal shock of pocket constituents, resealing of thepocket, and continued crystal growth or complete degassing, deposition of metastablefeldspar assemblages (high-sanidine, orthoclase, and intermediate microcline; R. Martinpers. com., 1983), and hermetical sealing of the pocket(any fluid entering the pocket

2. Complexly zoned, t y p i c a l l y l a r g e d i kes w i t h we l l - fo rmed m i a r o l i t i c c a v i t i e s . Dikes o f t h i s k i n d a re t y p i c a l l y l a r g e r than 10 x 10 x 0.2 meters and e x h i b i t w e l l - d e f i n e d zon ing w i t h w a l l zones, i n t e r m e d i a t e zones, and core zones. Whi le t h e w a l l zones a r e u s u a l l y p o o r l y de f ined , t h e i n t e r m e d i a t e zones g e n e r a l l y c o n s i s t o f g raph ic zones, b l o c k y zones ( u s u a l l y near co res ) and pocket zones o f ap l i t e . M inera logy i s d i ve r se : m i c r o c l i n e , a l b i t e , and q u a r t z a r e t h e e s s e n t i a l m ine ra l s , o f t e n o c c u r r i n g i n two o r more success ive generat ions. Accessory m ine ra l s i n c l u d e severa l o r numerous spec ies o f t h e f o l l o w i n g : s u l f i d e s - p y r i t e , s p h a l e r i t e , galena; s u l f o s a l t s - jamesoni te, boulange i t e and o t h e r i d e n t i f i e d spec ies; carbonates - s i d e r i t e (commonly rep laced by F e m ine ra l s 1 i ke goe th i t e , hemat i te , and 1 ep ido- c r o c i t e ) c a l c i t e , mangano-cal c i t e and a n k e r i t e ( ? ) ; Be-mineral s such as phenaki t e , b e r t r a n d i t e , baveni t e , euc l ase, and b e r y l ; and REE-ri ch (Th-poor) che ra l i t e , xenotime, monazi te, and s y n c h i s i t e - p a r i s i t e .

Dikes o f t h i s t ype commonly show t h e e f f e c t s o f thermal shock: i .e . b r e c c i a t e d pockets showing secondary overgrowths and cementat ion. I n a few cases t h e pockets were r a p i d l y evacuated o f a1 1 f l u i d s , and metas tab le f e l d s p a r assemblages c r y s t a l 1 - i z e d f rom escaping f l u i d s w h i l e c o a t i n g a l l e a r l i e r pocket phases.

3. Simple bodies w i t h ex tens i ve vuggy reg ions i n t h e i n t e r m e d i a t e zone. These d i kes a r e l a rqe , t h i c k , and do n o t show w e l l - d e v e l o ~ e d zoninq. Wall and i n t e r - mediate zones c o n t a i n p a t c h y qua r t z cores. Large m i a r o l i ti c c a v i t i e s a r e r a r e , b u t myr iads o f t i n y v e s i c l e s abound i n t h e i n t e r m e d i a t e zone. Maximum s i z e o f these c a v i t i e s i s about 0.5 - 1.5 cen t imete rs . Dominant m ine ra l s a re m i c r o c l i n e , a1 b i t e , quar tz , and t i n y (<1.0 m i l 1 i m e t e r ) hemat i te c r y s t a l s . Accessory m i n e r a l s i n c l u d e z i r c o n , f l u o r i t e , F -apa t i te . La te s tage m i l k y q u a r t z ve i ns c u t these d i kes i n about 50 percen t o f t h e cases.

4. Pegmati te bodies w i t h l a t e s tage s e l e c t i v e e t c h i n g and c r y s t a l l i z a t i o n o f accessory m inera ls . A l though s i m i l a r i n many respec ts t o t h e complex ly zoned l a r g e d i kes desc r i bed above, t h e y d i f f e r i n severa l s i g n i f i c a n t respec ts . They t y p i c a l l y show an inward d i m i n u t i o n i n qua r t z con ten t owing t o removal by c o r r o s i v e f l u i d s . Spaces o r i g i n a l l y f i l l e d w i t h q u a r t z were f i l l e d b y g raph i c i n t e r g r o w t h s o f q u a r t z and f e l dspa r . Secondary growth o f f e l dspars was accompanied by c r y s t a l 1 i z a t i o n o f T i ox ides (anatase b r o o k i t e r u t i l e ) and i l m e n i t e . Other m ine ra l s i n c l u d e T i b e a r i n g hemat i te ( T i = 2-3 pe rcen t ) , z i r con , muscovi te, b e r t r a n d i t e , phenak i te , c h e r a l i t e , xenotime, and o the rs .

I n t e r n a l E v o l u t i o n o f Megmatite Dikes (Types 2 and 4 above)

Development o f a ve r y t h i n , f i n e - g r a i n e d c o n t a c t zone (0.5 - 2 cen t ime te r s ) was f o l l owed b y c r y s t a l l i z a t i o n o f an ex tens i ve w a l l zone c o n t a i n i n g e l onga te b i o t i t e . Vapor nour ishment f e d c r y s t a l s growing down f rom hangingwal l . F i n e r g ra i ned p a r t s o f t h e w a l l zone grew upwards f rom t h e f o o t w a l l , w i t h occas iona l development o f a coarse-gra ined aggregate i n pockets o f vapor nour ishment. R i s i n g vapors i n i t i a l l y formed coarse K- fe ldspar i n g raph ic i n t e r g r o w t h w i t h quar tz , and l a t e r b l ockky K- fe ldspar . Qua r t z formed l a r g e monominera l ic masses i n t h e upper-median p a r t o f t h e d i ken A few v e r y l a r g e c r y s t a l s o f s i d e r o p h y l l i t e formed a t t h e co re margin. As t h e pocket s tage was approched a p ressure quench may have a f f e c t e d t h e f l u i d y i e l d i n g an a p l i t e , w h i l e i n o t h e r s e c t i o n s o f t h e d i ke , m i a r o l i t i c c a v i t i e s formed. These c a v i t i e s may be s i n g u l a r , r e l a t i v e l y l a r g e openings, o r t h e y may be ex tens i ve areas i n t h e i n t e r m e d i a t e zone o f t i n y v e s i c l e s w i t h m a t e r i a l resembl ing a sponge. A t l a t e r stages, metasomatism o r hydrothermal replacement o f some pocket cons i t uen t s s e t s i n - w i t h a l t e r a t i o n o f s i d e r i t e , p y r i t e , and o t h e r m ine ra l s t o g o e t h i t e , hemat i te , h y d r a t i o n o f phenaki t e t o b e r t r a n d i t e , c o r r o s i o n o f q u a r t z and f e l dspars (adu l a r i a - h a b i t ) , pocket r u p t u r e and thermal shock o f pocket c o n s t i t u e n t s , r e s e a l i n g o f t h e pocket, and con t inued c r y s t a l growth o r complete degassing, d e p o s i t i o n o f metas tab le f e l d s p a r assemblages ( h i gh-sanid ine, o r t h o c l ase, and i n t e r m e d i a t e m i c r o c l i n e ; R. M a r t i n pers. corn., 1983), and he rme t i ca l s e a l i n g o f t h e ~ o c k e t (any f l u i d e n t e r i n g t h e pocket

-16—

at this time would quickly react with the metastable phases.) Corrosive fluidsthen remove quartz from graphic intergrowths. uAlpine_typeu minerals are thendeposited in voids. The latest phase of pegmatite evolution involves theformation of clay minerals, etching of pocket constituents, and destruction dueto weathering (Jahns, 1955 and 1982, Foord and Martin, 1979, Martin, 1982,Cern9. 1982, Falster, 1983.)

(1) (2)?T z:;7\I,. /\ ,'-K

• )( • )< )( • )< x xX,('X,( X. . )( >./ ,t i ,'—/ t, — /\

(3) (4)

\ / —. — —/ \I' /"\I\—• :._r— . ,<

''x xX (X'. ..';'z( x

•• . . • .

t71/\/\/I/',/\ \J—//,/_\/_I I/\/N'' i,N\/—, //\

(1) Pegmatite schlieren: (2) Simple "vuggy" pegmatite dike: (3) Zonedpegmatite dike; (4) Zoned dike with solution-etched regions.

Explanation of Symbols

Aplite contact zone I 1 Coarse feldspar

L"S'i Wall Zone Pocket (or aplite unit)

Quartz Core

_____

Solution—etched region

_____

Coarse quartz-feldspar

Figure 10—— Types of pegmatite bodies in the Ninemile pluton.

The Ninemile granite itself shows much compositional and textural variation:miarolitic cavities and xenolithIc material are common, especially that portion ofthe Ninemile pluton occupying the southern rim of the Rib Mountain pluton, wheremuch stoped rim materials were never completely assimilated. Table 1 on page 16gives brief descriptions and modal compositions for 17 samples taken in N-S and E—Wtraverses across the pluton.

a t t h i s t i m e would q u i c k l y r e a c t w i t h t h e m e t a s t a b l e phases.) C o r r o s i v e f l u i d s t h e n remove q u a r t z f rom g r a p h i c i n t e r g r o w t h s . "A lp ine - t ype ' ' m i n e r a l s a r e t h e n d e p o s i t e d i n vo ids . The l a t e s t phase o f p e g m a t i t e e v o l u t i o n i n v o l v e s t h e fo rmat ion o f c l a y m i n e r a l s , e t c h i n g o f pocke t c o n s t i t u e n t s , and d e s t r u c t i o n due t o w e a t h e r i n g (Jahns, 1955 and 1982, Foord and M a r t i n , 1979, M a r t i n , 1982, tern;. 1982, F a l s t e r , 1983.)

( I ) Pegmat i te s c h l i e r e n : ( 2 ) S imple "vuggy" p e g m a t i t e d i k e : ( 3 ) Zoned pegmat i te d i k e ; ( 4 ) Zoned d i k e w i t h so1 u t i o n - e t c h e d r e g i o n s .

E x p l a n a t i o n o f Symbols

A p l i t e c o n t a c t zone 1-1 Coarse f e l d s p a r

L-] w a l l zone l z i Pocket ( o r a p l i t e u n i t )

pm Q u a r t z Core

I= Coarse q u a r t z - f e l dspar

So1 u t i o n - e t c h e d r e g i o n

F i g u r e l o - - Types o f pegmat i te bod ies i n t h e N i n e m i l e p l u t o n .

The N i n e m i l e g r a n i t e i t s e l f shows much c o m p o s i t i o n a l and t e x t u r a l v a r i a t i o n : m i a r o l i t i c c a v i t i e s and x e n o l i t h i c m a t e r i a l are common, e s p e c i a l l y t h a t p o r t i o n o f t h e N i n e m i l e p l u t o n occupy ing t h e s o u t h e r n r i m o f t h e R i b Mounta in p l u t o n , where much s toped r i m m a t e r i a l s were never c o m p l e t e l y a s s i m i l a t e d . Tab le 1 on page 16 g i v e s b r i e f d e s c r i p t i o n s and modal compos i t i ons f o r 17 samples t a k e n i n N-S and E-W t r a v e r s e s ac ross t h e p l u ton.

—17—

TABLE 1 —- MODAL COMPOSITION OF NINEMILE GRANITE PLUTON *

SAMPLENUMBER*

K-Feld Perth Plag Q Bi Am Rock Classification North

72395 28 24 12 35 Tr - Leucogranite72392A 12 51 - 31 2 — Biotite granite723928 62 - - 35 2 - Quartz syenite W/ xenocrystic quartz72391A 6 62 — 29 3 — Biotite granite72391B 10 55 — 21 — — Leucogranite72393 16 46? - 35 2 1 Gneissic qsy or granite aplite72394A 57 10 10 21 — - Hematitic quartz monzonite72394B 47 23 9 19 — - Quartz syenite or nnzonite w/ q veins72390A 32 3 30 27 0.6 2 Biotite quartz monzonite72390B - 60 5 28 4 1 Biotite granite72390C 28 12 21 32 7 — Biotite quartz monzonite72390D72389

3050?

1430?

8?5

3515

3<1

—

—

Altered biotite graniteLeucogranite

72388A 42? 16? 5 26 7 2 Hornblende—biotite granite723888 54 5 — 35 3 1 Biotite granite72387A 41 23 - 26 9 Tr Biotite granite723878 25 42 5 25 2 — Granite protomylonite7238672385

7029

5—

35

-37-10

2216- -

9--8

LeucograniteBiotite-hornblende quartz monzonite

Sotth'West

Hornblende-biotlte quartz moite72399

-— 40 5 10

72251 37 — 34 28 Tr — Leuco—quartz monzonite72400 - 65 — 31 2 — Granite protomylonite72401A72401B72401C72402A72402B729472403

—

—

-26—

10

I

7640—

—

-6460?

45

3446152515

152

2

—

60-23

22526

85

1

1

—

21

2816

—

-1

Biotite graniteNnphibole—biotite syenite (xennllth)Mafic syenogabbro (xenolith)Biotite syenite (xenolith)Schistose quartz—plagioclase apliteQuartz monzonite flaser gneisslQuartz monzonite

Sample Location Map

East

-1 7-

TABLE 1 -- MODAL COMPOSITION O F NINEMILE GRANITE PLUTON *

SAMPLE NUMBER*

72395 72392A 72392B . 72391A 72391 B 72393 72394A 72394B 72390A 72390B 72390C 723900 72389 72388A 723885 72387A 72387B 72386 72385

72399 72251 72400 72401A 72401B 72401C 72402A 72402B 7294 72403

K- Fel d 1 - - 28 12 62

6

e r t h

24 51

62 55 46? 10 23

3 60 12 14 30? 16? 5

23 42

5

-"- - - I

Sample Location Map

35 - 65 76 40 - - - 64 60?

- 1 i 7 1 16 1 9 1 8 1 B iot i te-hornblende quar tz monzonite 1

Plag

12 - - - - 10 9

30 5

21 8? 5 5 - - 5

10 34 - 4 5

34 46 15 25 15

Q 35 31 35 29 21 35 21 19 27 28 32 35 15 26 35 26 25 22

40 28 31 15 2 2 -

60 - 23

B i

T r 2 2 3 - 2 - -

0.6 4 7 3

7 3 9 2 -

5 T r

2 2

25 26

5 1 1

Am - - - - -

<1 - - 2 1 - - - 2 1 T r - -

10 - - - 21 28

8 1 6 - - 1

Rock C l a s s i f i c a t i o n North

Leucograni t e B i o t i t e g r a n i t e I Quartz syen i te w/ xenocrystic quar tz B i o t i t e g r a n i t e

s o h h west

Hornblende-biot i te quar tz moii%iiite Leuco-quartz m n z o n i t e Grani te pmtomy lon i te B i o t i t e g ran i te 1 Ampbi b o l e - b i o t i t e syeni t e (xennl i t h ) Maf ic syenogabbro (xeno l i th ) B i o t i t e syen i te (xeno l i th )

Quartz monzonite f l a s e r gneiss

I Schistose quar tz-p lagioc lase a p l i t e

Quartz mnzon i t e FA<+

Leucogranite Gneissic qsy o r g r a n i t e a p l i t e Hemat i t ic quar tz monzonite Quartz syen i te o r m n z o n i t e w/ B i o t i t e quar tz m n z o n i t e B i o t i t e g r a n i t e B i o t i t e quar tz m n z o n i t e A l te red b i o t i t e g r a n i t e Leucograni t e Hornblende-biot i te g ran i te B i o t i t e g r a n i t e B i o t i t e g r a n i t e Grani te pmtomy lon i te Leucograni t e

q veins

-18-

STOP #4

TITLE: Metaconglomerate inclusion(?) in quartz syenite in the west edge of theRib Mountain Pluton

LOCATION: NW/4 NE/4, Sec. 22, T 28 N, R 6 E, Marathon 15' Quadrangle [See map, p 8 ]

AUTHOR: Paul E. Myers, University of Wisconsin — Eau Claire

DATE: March, 1984

DESCRIPTION:

Large boulder piles south of the road contain numerous boulders of highlydeformed metaconglomerate which is cut by small dikes and veins of quartz syeniteand granite. Despite lack of outcrop here, the size, abundance, and uniquenessof this rock indicate that it was broken loose from a bedrock ledge by a farmerand hauled into this pile. Ninemile granite is exposed just east and south ofhere, and quartz syenite with abundant metasedimentary and metavolcanic xenolithssimilar to those seen at Stop #2 (Mosinee Hill) is exposed just east and north ofhere. There is good enough exposure in this area to indicate that there areseveral N-S and NE—SW trending faults which have broken the western edge of theRib Mountain pluton into slices. Thus, the contact relations of the rocks seen inthese rock piles, combined with distribution of rock types in the surrounding area,can be used to develop a conception of their relationships in spite of the lack ofexposure.

The metaconglomerate contains flattened and folded clasts of banded quartzite,fine-grained biotite schist and gneiss (felsic volcanic or metasediment?), meta-morphosed fine—grained andesitic(?) volcanic rocks with relict porphyritic,vesicular and tuffaceous textures, metadiorite, and metagabbro. One clast lookslike amphibole syenite! The clasts have been differentially flattened with simul-taneous development of a biotite—chlorite foliation. Quartzite clasts, becauseof resistance to flattening, remained quite round: the schistose metavolcanic clastswere greatly flattened, so that their length/thickness ratios are up to 12.(Figure 11). A later episode of close folding produced kink-like folds withconsequent second-stage deformation of the clasts.

Granitic dikes and veinlets cutting the metaconglomerate discordantly show apronounced reduction in grain size in the vicinity of the metacongomerate xenoliths -probably due to a change in the water content in the granite magma as a result ofstoping fo much wallrock. Some of the granite contains granules of quartz(xenocrysts?) up to 10 millimeters in diameter. K-feldspar in this granite ishighly perthitic. These relationships are compatible with the interpretation thatthe rocks here are near the contact between a xenolith-rich quartz syenite andthe younger Ninemile granite (1500 m.y.).

a Andesite

cs Chlorite schistfv Felsic volcanicspa Porphyritic andesiteq Quartzitemd Metadiorite

Figure 11-- Texture of deformed and metamorphosed polymictic conglomerate.

g

STOP #4

TITLE : Metaconglomerate inclusion(?) in quartz syenite in the west edge of the Rib Mountain Pl uton

LOCATION: NW14 NE/4Â Sec. 2 Z Y T 28 N y R 6 E y Marathon 15' Quadrangle [See mapy p 8 ]

AUTHOR: Paul E. Myersy University of Wisconsin - Eau Claire

DATE : Marchy 1984

DESCRIPTION:

Large boulder piles south of the road contain numerous boulders of highly deformed metaconglomerate which i s cut by small dikes and veins of quartz syenite and granite. Despite lack of outcrop herey the s i z e y abundancey and uniqueness of th i s rock indicate that i t was broken loose from a bedrock ledge by a farmer and hauled into th i s pile. Ninemile granite i s exposed just east and south of herey and quartz syenite with abundant metasedimentary and metavolcanic xenoliths similar to those seen a t S t o p #2 (Mosinee Hi l l ) i s exposed just east and north of here. There i s good enough exposure in th i s area t o indicate that there are several N-S and NE-SW trending fau l t s which have broken the western edge of the Rib Mountain pluton into s l ices . Thusy the contact relations of the rocks seen in these rock p i l e sy combined with distribution of rock types in the surrounding areay can be used to develop a conception of the i r relationships in sp i te of the lack of exposure.

The metacongl omerate contains f1 attened and fo1 ded c1 as t s of banded quartzite fine-grained b io t i te schis t and gneiss ( fe l s i c volcanic or metasediment?) metamorphosed fine-grained andes i t ic (?) vo1 canic rocks with re1 i c t porphyriticy vesicular and tuffaceous texturesy metadioritey and metagabbro. One c l a s t looks 1 i ke amphibole syeni te! The c las t s have been d i f fe rent ia l ly flattened with simul- taneous development of a biot i te-chlori te fol ia t ion. Quartzite c l a s t s y because of resistance to f la t teningy remained quite round: the schistose metavolcanic c las t s were greatly f ldt tenedy so that the i r lengthlthickness rat ios are u p to 12. (Figure 11 ). A l a t e r episode of close folding produced kink-like folds with consequent second-stage deformation of the c las t s .

Granitic dikes and veinlets cutting the metaconglomerate discordantly show a pronounced reduction in grain s ize in the vicini ty of the metacongomerate xenoliths - probably due t o a change in the water content in the granite magma as a resul t of stoping fo much wallrock. Some of the granite contains granules of quartz (xenocrysts?) u p t o 10 millimeters in diameter. K-feldspar in t h i s granite i s highly perthi t ic . These relationships are compatible with the interpretation that the rocks here are near the contact between a xenolith-rich quartz syenite and the younger Ninemil e granite (1 500 m.y.).

a Andesite

cs Chlorite schis t

fv Felsic volcanics

pa Porphyritic andesi t e

q Quartzite

md Metadiorite

Figure 11 - - Texture of deformed and metamorphosed polymicti c conglomerate.

—19—

The matrix is foliated and micaceous with lenticular fragments of quartzo-feldspathic rock (metavolcanic?), quartzite, and biotite. The clast lithologiesindicate deposition near the contact between a cratonic terrane mantled by quartziteand a volcanic terrane. The volcanic portion of the conglomerate resembles thoseexposed nearby: the quartzite, however has no present nearby source. It is possiblethat the volcanics were overlain by platform quartzites, eroded, and then deformedfirst by flattening with development of foliation and then by close folding(Figure 12). These types of deformation are not seen in the greenschist faciesmetavolcanic and plutonic rocks of central Marathon County — the region surroundingthe Wausau syenite complex.

Figure 12- Sequence of deformation in the metaconglomerate.Foliation was probably developed during initial flatteningof the quartzite clasts and later buckled by close-folding.Part of the chlorite was replaced by biotite during emplace-ment of the Rib Mountain pluton.

rocks showing these types of deformation and lithology favorthey represent materials carried upwards during syenite

Figure 13—— Zoned dikes of aplitic r'iineniile granite cuttingfoliated, schistose hiotite metaconalomerate from Loc. 72065

The absence ofthe conclusion thatemplacement.

The m a t r i x i s f o l i a t e d and micaceous w i t h l e n t i c u l a r f ragments o f quar tzo - f e l d s p a t h i c r ock (metavo1 c a n i c ? ) , q u a r t z i t e , and b i o t i t e . The c l a s t 1 i t h o l o g i e s i n d i c a t e d e p o s i t i o n near t h e c o n t a c t between a c r a t o n i c t e r r a n e mant led by q u a r t z i t e and a v o l c a n i c t e r r ane . The v o l c a n i c p o r t i o n o f t h e conglomerate resembles those exposed nearby: t h e q u a r t z i t e , however has no p resen t nearby source. I t i s p o s s i b l e t h a t t h e v o l c a n i c s were o v e r l a i n by p l a t f o r m q u a r t z i t e s , eroded, and then deformed f i r s t by f l a t t e n i n g w i t h development o f f o l i a t i o n and then by c l o s e f o l d i n g ( F i g u r e 12). These types o f de fo rmat ion a r e n o t seen i n t h e g reensch i s t f a c i e s metavo lcan ic and p l u t o n i c rocks o f c e n t r a l Marathon County - t h e r e g i o n su r round ing t h e Wausau s y e n i t e complex.

F i gure 12- Sequence o f de fo rmat ion i n t h e metaconglomerate. F o l i a t i o n was p robab ly developed d u r i n g i n i t i a l f l a t t e n i n g o f t h e q u a r t z i t e c l a s t s and l a t e r buck led by c l o s e - f o l d i n g . P a r t o f t h e c h l o r i t e was rep laced by b i o t i t e d u r i n g emplacement o f t h e R ib Mountain p l u ton .

The absence o f rocks showing these types o f de fo rmat ion and l i t h o l o g y f a v o r t h e conc lus ion t h a t t h e y r ep resen t m a t e r i a l s c a r r i e d upwards d u r i n g s y e n i t e empl acement .

F igu re 13-- Zoned d i kes o f a p l i t i c N inemi le g r a n i t e c u t t j n g f o l i a t e d , sch i s t ose b i o t i t e wetaconalomerate f rom LOC. 72065

-20—

STOP #5

TITLE: Border Phases of the Wausau Pluton

LOCATION: Wisconsin River at Old Technical Institute, Wausau. NE¼, NE¼, Sec. 35,T29N, R7E. Wausau 15' and Wausau West 7½' quadrangles.

Lfi'i,Ik1

flIiI -

R rirp •

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AUTHOR: Paul E. Myers, University of Wisconsin - Eau ClaireDATE: March, 1984


Flow-lineated amphibole syenite containing sheet-like masses of syenitizedsiliceous metavolcanic rocks and mafic schist as well as blocky xenoliths ofbiotite amphibolite and metavolcanic rocks resembling those at Brokaw is wellexposed along the overflow channel of the power dam in Wausau. Xenolith orient-ation is northwesterly here. The syenite is cut by zoned, rootless, steeplydipping pegmatite veins with quartz cores. Late fracture-filling veinlets ofreibeckite are locally abundant.DESCRIPTION:

Weidman's (1907, p. 203-208) tWausau-type quartz syenite is composed ofalkali feldspars: orthoclase, microcline, albite, and microperthite and lessabundant barkevikite, hedenbergite, fayalite, biotite, and quartz. Accessoryminerals include fluorite, apatite, magnetite, zircon, and allanite(?).

Cross-cutting relationships here typify those seen throughout the Wausaupluton. Major components are described below in order of decreasing age.

1. The oldest rocks are xenoliths of schistose biotite amphibolite andslightly altered felsic metavolcanics resembling those at Brokaw.Sheet—like masses of siliceous and foliated mafic rocks occur on thesouth end of the exposure (Figure 14, Loc. C). These inclusions havea pronounced northwesterly strike and nearly vertical dip. This fabricis consistent with the concentric structure of the Wausau pluton north-west of here. A highly altered mafic xenolith at Location A (Figure 15)has swirled foliation and is cut by rootless pegmatite veins with K—feld-spar walls and quartz cores. Some of the mafic volcanic xenoliths haverelict porphyritic textures: mafic xenoliths are now biotite-rimmedsubhedral crystals of hastingsite and ferrohastingsite (Figure 15,Loc. B).

8

4.

STOP #5

TITLE: Border Phases o f t h e Wausau P l u t o n

LOCATION: Wisconsin R i v e r a t O ld Technica l I n s t i t u t e y Wausau. NEkY NEkY Sec. 3 s y T29Ny R7E. Wausau 15 ' and Wausau West 7%' quadrangles.

AUTHOR: Paul E. Myersy U n i v e r s i t y o f Wisconsin - Eau C l a i r e

DATE : Marchy 1984


F l ow- l i nea ted amphibole s y e n i t e c o n t a i n i n g s h e e t - l i k e masses o f s y e n i t i z e d s i l i c e o u s metavo lcan ic rocks and m a f i c s c h i s t as w e l l as b l o c k y x e n o l i t h s o f b i o t i t e a m p h i b o l i t e and metavo lcan ic rocks resembl ing those a t Brokaw i s we11 exposed a l ong t h e o v e r f l o w channel o f t h e power dam i n Wausau. X e n o l i t h o r i e n t - a t i o n i s n o r t h w e s t e r l y here. The s y e n i t e i s c u t by zonedy r o o t l e s s y s t e e p l y d i p p i n g pegmat i te ve ins w i t h qua r t z cores. La te f r a c t u r e - f i l l i n g v e i n l e t s o f r e i b e c k i t e a r e l o c a l l y abundant.

DESCRIPTION:

Weidman's (1 907y p. 203-208) "Wausau-typet' q u a r t z syen i t e i s composed o f a l k a l i f e l dspa rs : o r t hoc lase , m i c r o c l i n e y a l b i t e y and m i c r o p e r t h i t e and l e s s abundant b a r k e v i k i t e y hedenberg i tey f a y a l i t e , b i o t i t e y and quar tz . Accessory m ine ra l s i n c l u d e f l u o r i t e a p a t i t e y magnet i t e y z i r c o n and a1 l a n i t e ( ? ) .

C ross - cu t t i ng r e l a t i o n s h i p s here t y p i f y those seen th roughou t t h e Wausau p l u ton . Ma jo r components a re desc r i bed below i n o r d e r o f dec reas ing age.

The o l d e s t rocks a re x e n o l i t h s o f s c h i s t o s e b i o t i t e amph ibo l i t e and s1 i g h t l y a1 t e r e d f e l s i c metavo lcan ics resembl i n g those a t Brokaw. S h e e t - l i k e masses o f s i l i c e o u s and f o l i a t e d m a f i c rocks occur on t h e sou th end o f t h e exposure ( ~ i g u r e 1 4 y LOC. C). These i n c l u s i o n s have a pronounced n o r t h w e s t e r l y s t r i k e and n e a r l y v e r t i c a l d i p . Th i s f a b r i c i s c o n s i s t e n t w i t h t h e c o n c e n t r i c s t r u c t u r e o f t h e Wausau p l u t o n n o r t h - west o f here. A h i g h l y a l t e r e d m a f i c x e n o l i t h a t Loca t i on A ( F i g u r e 15) has s w i r l e d f o l i a t i o n and i s c u t by r o o t l e s s pegmat i te ve i ns w i t h K - f e l d - spar w a l l s and qua r t z cores. Some o f t h e m a f i c v o l c a n i c x e n o l i t h s have re1 i c t p o r p h y r i t i c t e x t u r e s : m a f i c x e n o l i t h s a re now b i o t i t e - r i m m e d subhedral c r y s t a l s o f h a s t i n g s i t e and f e r r o h a s t i n g s i t e ( ~ i gure 15 LOC. 0 ) .

Figure 14—— Sketch map showinglocations of points of interest

Figure 15-- Amphibolite (a) xenolith with swirledlineation and thin seams of syenite (Loc. A, Fig.14). Pegmatitic veins have K-feldspar (Kf) coresand quartz (q) cores. Late fracture-filling veinsare filled with coarse laths of sodic amphibole.Biotite and amphibole in amphibolite are aboutthe same composition as those in the enclosing,flow—lineated syenite (determined by microprobe).Bar scale in sketch is 6 inches long. View isoblique toward northwest.

2. An early, flow-laminated, foliated quartz syenite containing lenticularxenoliths of porphyritic felsic volcanic rocks and biotite amphibolitewith relict mafic phenocrysts is exposed at Location B.

3. Coarse—grained, flow-lineated amphibole quartz syenite cuts the fine-grainedphase (Loc. B) with sharp discordance. Although the lensoidal and tabularinclusions in this rock show a northwesterly orientation, the enclosingsyenite shows highly discordant flow lineation with swirls and eddiessuggesting considerable turbulence and viscosity. Considering the lack ofdeformation in the sheets of biotite amphibolite and siliceous metavolcanicrock in this syenite, a model requiring turbulent flow in the enclosingsyenite magma runs into trouble. The appearance of discordance may be pro-duced by obliquity of lineation with respect to the eroded rock surface.

4. Late—stage, rootless" granite pegmatite veins with quartz cores probablyrepresent residual liquid segregations along incipient thermal contractionfractures in the crystallized syenite. The pegniatites may derive fromunassimilated siliceous rocks, possibly feldspathized quartzites.

5. Coarse, sodic amphibole (arfvedsonite and riebeckite) crystallized alongfracture surfaces1and appear to cut all other structures.

At Location B, mafic and felsic xenoliths are aligned N1OW, vertical in amedium-grained tabular syenite with a fabric similar to that seen in the marginof the Stettin pluton. This rock is cut discordantly by coarse gray amphibole

—21-

4N

0

1•>

F i g u r e 14-- Sketch map showing

F i g u r e 15-- Amphibol i t e ( a ) xenol i t h w i t h s w i r l e d l i n e a t i o n and t h i n seams o f s y e n i t e (Loc. A, F ig . 14). Pegmati t i c v e i n s have K - f e l dspar ( K f ) co res and q u a r t z ( q ) cores. L a t e f r a c t u r e - f i l 1 i n g v e i n s a r e f i l l e d w i t h coarse l a t h s o f s o d i c amphibole. B i o t i t e and amphibole i n a m p h i b o l i t e a r e abou t t h e same c o m p o s i t i o n as those i n t h e e n c l o s i n g , f l ow-1 i n e a t e d s y e n i t e (de te rm ined by m ic rop robe) . Bar s c a l e i n s k e t c h i s 6 i nches l o n g . View i s

l o c a t i o n s o f p o i n t s o f i n t e r e s t o b l i q u e toward no r thwes t .

An e a r l y , f l ow-1 aminated, f o l i a t e d q u a r t z s y e n i t e c o n t a i n i n g l e n t i c u l a r x e n o l i t h s o f p o r p h y r i t i c f e l s i c v o l c a n i c r o c k s and b i o t i t e a m p h i b o l i t e w i t h r e l i c t m a f i c phenocrys ts i s exposed a t L o c a t i o n B.

Coarse-grained, f l o w - l i n e a t e d amphibole q u a r t z s y e n i t e c u t s t h e f i n e - g r a i n e d phase (Loc. B ) w i t h sharp d iscordance. A l though t h e l e n s o i d a l and t a b u l a r i n c l u s i o n s i n t h i s r o c k show a n o r t h w e s t e r l y o r i e n t a t i o n , t h e e n c l o s i n g s y e n i t e shows h i g h l y d i s c o r d a n t f l o w l i n e a t i o n w i t h s w i r l s and edd ies s u g g e s t i n g c o n s i d e r a b l e t u r b u l e n c e and v i s c o s i t y . C o n s i d e r i n g t h e l a c k o f de fo rmat ion i n t h e shee ts o f b i o t i t e a m p h i b o l i t e and s i l i c e o u s m e t a v o l c a n i c r o c k i n t h i s s y e n i t e , a model r e q u i r i n g t u r b u l e n t f l o w i n t h e e n c l o s i n g s y e n i t e magma runs i n t o t r o u b l e . The appearance o f d i sco rdance may be p ro - duced b y o b l i q u i t y o f l i n e a t i o n w i t h r e s p e c t t o t h e eroded r o c k s u r f a c e .

Late-s tage, r o o t l e s s " g r a n i t e pegmat i te v e i n s w i t h q u a r t z co res p r o b a b l y r e p r e s e n t r e s i d u a l l i q u i d s e g r e g a t i o n s a l o n g i n c i p i e n t the rma l c o n t r a c t i o n f r a c t u r e s i n t h e c r y s t a l l i z e d s y e n i t e . The pegmat i tes may d e r i v e f rom unass imi l a t e d s i 1 i c e o u s rocks, p o s s i b l y f e l d s p a t h i z e d q u a r t z i t e s .

Coarse, s o d i c amphibole (a r f vedson i t e and r i e b e c k i t e ) c r y s t a l 1 i z d d a l o n g f r a c t u r e s u r f a c e s a n d appear t o c u t a l l o t h e r s t r u c t u r e s .

L o c a t i o n B, m a f i c and f e l s i c x e n o l i t h s a r e a l i g n e d NlOW, v e r t i c a l i n a medium-grained t a b u l a r s y e n i t e w i t h a f a b r i c s i m i l a r t o t h a t seen i n t h e marg in o f t h e S t e t t i n p l u t o n . T h i s r o c k i s c u t d i s c o r d a n t l y b y coarse g r a y amphibo le

-22.-syenite. Thin, lenticular a.plite veinlets have an orientation of N75°W, 40°SThese shallow dips are common in granitic pods and veinlets in the syenite.

The sheet-like, foliated biotite amphibolite at Location C has the appearanceof a dike (Figure 16), but its contacts are highly convoluted, and foliationis deformed, although it is also locally cut by the syenite. The cuspateedge of the amphibolite sheet has whisps which tail out into the syenite(Figure 17). The trace of lineation on the rock surface gives the impressionof sharp discordance between the amphibolite sheet and the lineated syenite.Irregularities in the shape of the syenite—amphibolite contact can give theimpression that the amphibolite contains syenite inclusions (Figure 18).The same amphibolite sheet on the east side of the channel at Location D isbroken into angular fragments which can be fitted back together again. Itis therefore apparent that the mechanical behavior of the amphibolite in thesyenite varied significantly over short distances.

North

Figure 16-- Deformed, foliated biotite amphibolite sheet in amphi-bole syenite at Location C. Is it a dike which was intruded at anearly stage of syenite crystallization, then carried to itspresent location during intrusion of a syenitic crystal mush?

Slight, but probably significant differences in mineral chemistry can be seenwhen amphibole and biotite compositions in various inclusions are compared to theircomposition in the coarse and fine grained varieties of the amphibole syenite.Table 2 is a compilation of microprobe date for these minerals as well asplagioclase, which is a persistent accessory mineral in the syenite and in theinclusions.

t —

-22. syen i t e . Thin, l e n t i c u l a r a p l i t e v e i n l e t s have an o r i e n t a t i o n o f N75OW, 40Â° These sha l l ow d i p s a re common i n g r a n i t i c pods and v e i n l e t s i n t h e syen i t e .

The s h e e t - l i k e , f o l i a t e d b i o t i t e amph ibo l i t e a t Loca t i on C has t h e appearance o f a d i k e (F i gu re 16) , b u t i t s con tac t s a r e h i g h l y convolu ted, and f o l i a t i o n i s deformed, a l t hough i t i s a l s o l o c a l l y c u t b y t h e syen i t e . The cuspate edge o f t h e amph ibo l i t e sheet has whisps which t a i l o u t i n t o t h e s y e n i t e (F i gu re 17). The t r a c e o f l i n e a t i o n on t h e rock su r f ace g i ves t h e impress ion o f sharp d iscordance between t h e amph ibo l i t e sheet and t h e l i n e a t e d syen i t e . I r r e g u l a r i t i e s i n t h e shape o f t h e syen i t e -amph ibo l i t e c o n t a c t can g i v e t h e impress ion t h a t t h e amphibol i t e con ta i ns s y e n i t e i n c l u s i o n s (F i gu re 18). The same a m p h i b o l i t e sheet on t h e eas t s i d e o f t h e channel a t Loca t i on D i s broken i n t o angu la r fragments which can be f i t t e d back t o g e t h e r again. It i s t h e r e f o r e apparent t h a t t h e mechanical behav io r o f t h e amph ibo l i t e i n t h e s y e n i t e v a r i e d s i g n i f i c a n t l y ove r s h o r t d i s tances .

North

*

F i gure 16-- Deformed, f o l i a t e d b i o t i t e amphi bo l i t e sheet i n amphi- b o l e s y e n i t e a t Loca t i on C. I s i t a d i k e which was i n t r u d e d a t an e a r l y s tage o f s y e n i t e c r y s t a l l i z a t i o n , then c a r r i e d t o i t s p resen t l o c a t i o n d u r i n g i n t r u s i o n o f a s y e n i t i c c r y s t a l mush?

S l i g h t , b u t p robab ly s i g n i f i c a n t d i f f e r e n c e s i n m ine ra l chem is t r y can be seen when amphibole and b i o t i t e composi t ions i n va r i ous i n c l u s i o n s a re compared t o t h e i r composi t ion i n t h e coarse and f i n e g ra i ned v a r i e t i e s o f t h e amphibole syen i t e . Table 2 i s a c o m p i l a t i o n o f microprobe da te f o r these m ine ra l s as w e l l as p l ag ioc l ase , which i s a p e r s i s t e n t accessory m ine ra l i n t h e s y e n i t e and i n t h e i n c l us ions.

—23-

Figure 17-- Cuspate edge of foliated, sheet—like massof biotite amphibolite in lineated amphibole syenite.Location C. Bar scale is 6 inches long.

Figure 18—- Pseudoxenolith of syenite in biotite amphib-olite at Location C.

Figure 17-- Cuspate edge of fol ia ted, sheet-1 ike mass of b io t i te amphibolite in lineated amphibole syenite. Location C . Bar scale i s 6 inches long.

Figure 18-- Pseudoxenol i t h of syenite in b io t i te amphib- 01 i t e a t Location C.

-24-

Figure 19—— Segmented biotite amphibolite sheet inlineated, coarse-grained amphibole syenite. Location D.Rule is six inches long.

ELECTRON MICROPROBE

TABLE 2ANALYSES OF MAJOR MINERALS IN SYENITES AND INCLUSIONS AT FIELD TRIP STOP #5

AMPHIBOLES

Si02 Ti02 A12O3 FeO +Fe2 03

MgO CaO MoO Na20 K20 TOTAL DESCRIPTION

A/4*

B/3

D/6

G/5

H/2

43.30

42.07

43.20

42.20

41.32

1.44

1.40

1.54

1.77

1.36

6.64

6.32

6.35

8.14

8.10

28.84

32.55

29.63

25.92

29.04

3.60

1.56

2.75

5.03

2.76

9.93

9.28

9.72

10.79

10.63

0.99

0.69

0.79

0.62

0.81

2.24

2.24

2.32

2.12

1.89

1.03

1.16

1.14

1.23

1.32

98.01

97.27

97.44

97.82

97.23

Green amphibole in foliated biotiteamphibolite xenolith in syeniteMedium—grained. early phase amphi-bole syenite: green amphiboleolive green to olive brown amphi-bole in lineated amphibole syenitegreen amphibole in brown biotiteclots or plagioclaseGreen amphibole in lineated, coarse—grained amphibole syenite, late phase

BIOTI

A/2

8/1

D/4

G/5

HI?

TES

35.54

35.29

36.39

35.96

35.64

3.77

3.34

2.36

3.62

3.49

11.67

11.43

10.96

12.36

12.82

31.07

35.77

32.87

27.14

30.74

4.34

1.93

3.65

6.70

3.91

0.02

0.04

0.14

0.02

0.02

0.45

0.35

0.27

0.34

0.52

0.04

0.00

0.01

0.06

0.02

9.10

8.93

8.98

9.21

9.19

96.00

98.08

95.84

95.41

96.35

Yellow—brown with metamict zirconin green amphibole: amphibolite xeno.Olive brown with apatite in greenamphibole clustersYellow—brown anhedral biotite withfluorite in green amphiboleYellow—orange to orange—brown foliain biotie—amphibole schistRagged yellow—brown with green,anhedral amphibole and fluorite

PLAGIOCLASE FELDSPAR

D/2: 1.73%An G/3: 10.24-18.41%An Avg. 13.12%An H/3: 8.83-l2.44%An Zoned Plag. = 1O.68—8.67%An.

Sample Locations: Samples A and B are from Location B in Figure 14: Sample G is from Location C, and H is fromLocation D in Figure 14.

* Field number/number of analyses averaged

F i g u r e 19-- Segmented b i o t i t e a m p h i b o l i t e shee t i n l i n e a t e d , coarse -g ra ined amphibole s y e n i t e . L o c a t i o n D. Ru le i s s i x i n c h e s long .

T A B L E 2 ELECTRON MICROPROBE ANALYSES OF MAJOR MINERALS I N SYENITES AND INCLUSIONS AT FIELD TRIP SFOP #5

WPHIBOLES

BIOTITES

A12 35.54 I

FeO + MgO CaO MnO

Fe203

28.84 3.60 9.93 0.99

32.55 1.56 9.28 0.69

29.63 2.75 9.72 0.79

25.92 5.03 10.79 0.62

29.04 2.76 10.63 0.81

PLAGIOCLASE FELDSPAR

0/2: 1.73%An G/3: 10.24-18.4l%An Avg. 13.12%An

rOTAL I DESCRIPTION

38.01

37.27

97.44

97.82

97.23

H/3: 8.83-12.44%An Zoned Plag. = 10.68-8.67%An.

Green amphibole i n f o l i a t e d b i o t i t e amph ibo l i t e x e n o l i t h i n s y e n i t e

Medium-grained. e a r l y phase amphi- b o l e s y e n i t e : green amphibole

o l i v e green t o o l i v e brown amphi- b o l e i n l i n e a t e d amphibole s y e n i t e

green amphibole i n brown b i o t i t e c l o t s o r p l a g i o c l a s e

Green amphibole i n l i n e a t e d , coarse- g ra ined amphibole syen i t e , l a t e phase

96.00

98.08

95.84

95.41

96.35

Sample Loca t i ons : Samples A and B a r e from Loca t i on B i n F igu re 14: Sample G i s f rom L o c a t i o n C , and H i s f rom Loca t i on 0 i n F igu re 14.

I Yellow-brown w i t h metamic t z i r c o n i n green amphibole: a m p h i b o l i t e xeno.

O l i v e brown w i t h a p a t i t e i n green amphibole c l u s t e r s

Yellow-brown anhedra l b i o t i t e w i t h f l u o r i t e i n green amphi b o l e

Y e l l ow-orange t o orange-brown fo1 i a i n b i o t i e -amph ibo le s c h i s t

Ragged ye l low-brown w i t h green, anhedral amphibole and f l u o r i t e

* F i e l d number/number o f ana lyses averaged

—25—

STOP #6

TITLE: Contaminated Amphibole Quartz Syenite — Wausau Pluton

LOCATION: Old quarry behind Employers' Mutual Insurance Company offices,NW/4, SE/4, Sec. 27, T 29 N, R 7 E; Wausau West 7.5' Quadrangle

AUTHOR: Paul E. Myers, University of Wisconsin — Eau ClaireDATE: April, 1984DESCRIPTION:

Coarse pink and brownish gray amphibole quartz syenite is well exposed inrelatively fresh faces of an old quarry along a road leading into the offices ofEmployers' Mutual Insurance Company. Four facies were recognized: (1) brownishgray quartz—bearing syenite, (2) coarse, dark gray amphibole syenite, (3) pinkquartz syenite with abundant volcanic xenoliths, and (4) medium—grained pinkishbrown to brownish gray amphibole quartz syenite with magnetite segrations.(See Table 3 ). The pink syenites have a distinctly higher Fe3/Fe ratio thanthe gray syenites Magnetite sheets and irregular masses occur in the medium—grained syenite along the road on the east side of the outcrop. The magnetite-bearing quartz syenite forms a large, crescentic aeromagnetic anomaly on the mapby Henderson, Tyson, and Page, (1963). The anomaly is concentric and concordantwith the structure of the Wausau pluton.

The concentric structure of this pluton is accentuated by the occurrencejust north of here on the tree—covered hillside of numerous, large quartzitexenoliths. Unlike the Rib Mountain quartzite xenolith, this mass comprises manysmaller quartzite blocks. Orientation of xenoliths here and elsewhere in theWausau pluton is concentric and nearly vertical — a factor strongly suggestingsubvolcanic emplacement with successive collapse and intrusion of magma intoconcentric fracture systems of the caldera rim. The xenoliths at this locationare quite unlike those anywhere else in the Wausau pluton: they consist almostentirely of felsic volcanics showing virtually no pre-intrusion metamorphism.(See Figure 20 ).

Figure 20-- Felsic volcanic xenoliths (dotted) in flow-lineatedamphibole quartz syenite.

0meter (n detafled area)

-25- STOP #6

TITLE : Contaminated Amphibole Quartz Syenite - Wausau Pluton

LOCATION: Old quarry behind Employers1 Mutual Insurance Company off ices , NW/4, SE/4, Sec. 27 , T 29 N , R 7 E ; Wausau West 7.5' Quadrangle

A U T H O R : Paul E . Myers, Universi tyofWisconsin- Eau Claire

DATE : - Apri 1 , 1984 DESCRIPTION:

Coarse pink and brownish gray amphibole quartz syenite i s we relat ively fresh faces of an old quarry along a road leading into Employers' Mutual Insurance Company. Four facies were recoanized

11 exposed in the offices of

. - : ( 1 ) brownish gray quartz-bearing syeni t e , (2) coarse, dark gray amphi bole syenite, ( 3 ) pink quartz syeni t e with abundant vol canic xenol i ths , and ( 4 ) medium-grained pinkish brown t o brownish gray amphibole quartz syenite with magnetite segr ations. (See Table 3 ) The pink syeni tes have a d is t inc t ly higher Fe3+/Fe" ra t io than the gray syenites Magnetite sheets and irregular masses occur in the medium- grained syenite along the road on the east side of the outcrop. The magnetite- bearing quartz syenite forms a large, crescentic aeromagnetic anomaly on the map by Henderson, Tyson, and Page, (1963). The anomaly i s concentric and concordant with the structure of the Wausau pluton.

The concentric structure of th i s pluton i s accentuated by the occurrence just north of here on the tree-covered h i l l s ide of numerous, large quartzite xenoliths. Unlike the Rib Mountain quartzite xenolith, t h i s mass comprises many smaller quartzite blocks. Orientation of xenoliths here and elsewhere in the Wausau pluton i s concentric and nearly vertical - a factor strongly suggesting subvolcanic emplacement with successive collapse and intrusion of magma into concentric fracture systems of the caldera rim. The xenoliths a t t h i s location are quite unlike those anywhere e l se in the Wausau pluton: they consist almost ent i rely of f e l s i c volcanics showing vir tual ly no pre-intrusion metamorphism. (See Figure 20 ).

Figure 20-- Fel s i c volcanic xenol i t h s (dotted) in flow-1 ineated amphibole quartz syenite.

-26—

TABLE 3

Bulk chemical compositions of the four principal quartz syenite facies fromEmployers' Mutual Insurance Company Quarry.

EW-3 EW-5 1 NSI SEI(WEST) (EAST) (SOUTH) (NORTH)

Description Brownish- Coarse, dark Pink syenite, with Medium-grained— gray gray volcanic xenoliths syenite

Si02 63.05 63.55 63.90 64.10

Ti02 0.78 0.54 0.47 0.48

A1203 12.60 15.16 14.14 15.17

Fe203 1.91 1.25 5.42 4.58

FeO 7.72 3.48 1.32 1.44

MnO 0.34 0.16 0.14 0.12

MgO 0.41 0.16 0.45 0.09

CaO 2.66 1.72 1.35 1.50

Na2O 4.80 5.52 6.32 5.17

1(20 4.22 5.67 6.34 5.57

H20 0.76 0.42 0.56 0.26

P205 0.22 0.06 0.05 0.06

CO2 0.28 1.92 0.62 0.09

BaO 0.094 0.066 0.024 0.036

Zr02 0.222 0.114 0.062 0.071

Rb154 118 80 80

Sr ppm78 83 67 42

In comparison with Nockolds' (1954) average syenite composition (seeTable ), these quartz syenites are richer in Si09 and total iron and poorin alkalies and lime. Their Rb and Sr contents a'e also low compared toother similar rocks.

* From Sood, Myers, and Berlin, 1980, p. 21

TABLE 3

Bulk chemical compositions o f the f o u r p r i n c i p a l quar tz syeni t e f a c i e s from Employers' Mutual Insurance Company Quarry.

Descr ipt ion - S i 0,

n 0,

Ãˆ,O

FeO

In0

w CaO

1a20

K2Â

+zO

;02

BaO

Zr02

Rb s r P P ~

EN-3 (WEST)

Browni sh- gray

EN-5 ( EAST)

Coarse, dark gray

N S I (SOUTH)

Pink syeni te, w i t h vo l cani c xenol i t h s

SE I (NORTH)

Medi urn-grai nec syeni t e

I n comparison w i t h Nockol ds ' (1954) average syeni t e composi t i o n (see Table ), these quar tz syeni tes are r i c h e r i n S i O and t o t a l i r o n and poor

o ther s i m i l a r rocks. 6 i n a l k a l i e s and l ime. The i r Rb and S r contents a e a l s o low compared t o

* From Sood, Myers, and B e r l i n , 1980, p. 21

—27-

STOP #7

TITLE: Quartz-sillimanite-muscovite schist and quartzite xenoliths inquartz syenite of the Wausau pluton.

LOCATION: SE¼ , NE¼, T29N, R7E; Wausau 15' quadrangle [Figure 6]

AUTHOR: Paul E. Myers, University of Wisconsin - Eau ClaireDATE: March, 1984

DESCRIPTION:

Xenoliths in the Wausau syenite pluton are mainly syenitized mafic and inter-mediate metavolcanic rocks. Felsic metavolcanic rocks are locally abundant, asat the old Technical School (Stop 5), and may represent portions of the volcaniccover sequence which collapsed and were stoped by the rising syenite magmas.Mica schist and quartzite are commonly associated in the intermediate zones ofboth the Wausau and Rib Mountain plutons and may represent samples of basementrocks which underlie the volcanic rocks. Considerable variation in lithology,size, shape, and relative abundance of xenoliths suggests mixing of basementand cover rocks by complex subsidence and resurgence of the syenite magmas atshallow depth. Not only is the concentric structure of these plutons expressedby compositional layering and orientation of xenoliths, but xenolithology showsa crude concentric zoning within each luton.

The dominant lithology of xenoliths at this locality is sillimanite—bearinglineated, quartz-muscovite schist with a pronounced tectonite fabric (Figure 21).

Figure 21—— Photomicrograph (with half—crossed polars)of quartz—sillimanite-muscovite schist. Elongated,polygonal quartz grains show similar extinction positionsand elongation parallel to sillimanite tufts. Width ofphotomicrograph is 4.0 mm.

A nearby xenolith is coarse—grained which metaquartzite with sutured grains andno strain lamellae. Sillimanite is absent. This rock closely resembles the meta—quartzites of Rib Mountain. The strong contrast in the textures and mineral compo-sitions of these two rocks suggests widely different origins. Although sillimanitecan form metasomatically in metasomatic aureoles of granitic plutons, itsoccurrence here in a schistose rocks with a distinct tectonite fabric tends torule out that possibility. (See Figure 22.)

STOP #7

TITLE : Quartz-sillimanite-muscovite s c h i s t and q u a r t z i t e x e n o l i t h s i n q u a r t z s y e n i t e o f t h e Wausau p l u ton .

LOCATION: SEk , NEk, T29N, R7E; Wausau 15 ' quadrangle [F igure 6 1 AUTHOR : Paul E. Myers, U n i v e r s i t y o f Wisconsin - Eau C l a i r e

DATE : - March, 1984 DESCRIPTION :

Xeno l i t hs i n t h e Wausau s y e n i t e p l u t o n a re m a i n l y s y e n i t i z e d m a f i c and i n t e r - mediate metavol can i c rocks. Fe l s i c metavol c a n i c rocks a re l o c a l l y abundant, as a t t h e o l d Technica l School (Stop 5), and may rep resen t p o r t i o n s o f t h e v o l c a n i c cover sequence which co l l apsed and were s toped by t h e r i s i n g s y e n i t e magmas. Mica s c h i s t and q u a r t z i t e a r e commonly assoc ia ted i n t h e i n t e r m e d i a t e zones o f bo th t h e Wausau and R ib Mountain p l u tons and may rep resen t samples o f basement rocks which under1 i e t h e v o l c a n i c rocks. Cons iderab le v a r i a t i o n i n 1 i tho logy , s i ze , shape, and re1 a t i v e abundance o f xenol i t h s suggests m i x i n g o f basement and cover rocks by complex subsidence and resurgence o f t h e s y e n i t e magmas a t sha l low depth. Not o n l y i s t h e c o n c e n t r i c s t r u c t u r e o f these p l u tons expressed by compos i t i ona l l a y e r i n g and o r i e n t a t i o n o f x e n o l i t h s , b u t x e n o l i t h o l o g y shows a crude c o n c e n t r i c zon ing w i t h i n each b l u ton .

The dominant l i t h o l o g y o f x e n o l i t h s a t t h i s l o c a l i t y i s s i l l i m a n i t e - b e a r i n g 1 inea ted , quar tz-muscov i te s c h i s t w i t h a pronounced t e c t o n i t e f a b r i c (F i gu re 21 ) .

F i gure 21 -- Photomicrograph ( w i t h ha1 f -c rossed po l a r s ) o f quartz-sillimanite-muscovite s c h i s t . Elongated, po lygonal qua r t z g r a i n s show s i m i l a r e x t i n c t i o n p o s i t i o n s and e l o n g a t i o n p a r a l l e l t o s i l l i m a n i t e t u f t s . Width o f photomicrograph i s 4.0 mm.

A nearby x e n o l i t h i s coarse-gra ined which m e t a q u a r t z i t e w i t h su tu red g r a i n s and no s t r a i n lame l lae . S i l l i m a n i t e i s absent. Th i s r ock c l o s e l y resembles t h e meta- q u a r t z i t e s o f R ib Mountain. The s t r o n g c o n t r a s t i n t h e t e x t u r e s and minera l compo- s i t i o n s o f these two rocks suggests w i d e l y d i f f e r e n t o r i g i n s . A l though s i l l i m a n i t e can form me tasoma t i ca l l y i n metasomatic aureo les o f g r a n i t i c p l u tons , i t s occurrence here i n a s c h i s t o s e rocks w i t h a d i s t i n c t t e c t o n i t e f a b r i c tends t o r u l e o u t t h a t p o s s i b i l i t y . (See F igu re 22.)

-28-

Figure 22-- Photomicrograph of quartz—sillimanite schist (A)and metaquartzite (B). Note sutured grain boundaries andlack of sillimanite in B. Width of image is 1.5 mm.

F i g u r e 22 -- Photomicrograph o f q u a r t z - s i l l i m a n i t e s c h i s t ( A ) and m e t a q u a r t z i t e ( B ) . Note s u t u r e d g r a i n boundar ies and l a c k o f s i l l i m a n i t e i n B. Wid th o f image i s 1.5 mm.

—29—

THE STETTIN SYENITE PLUTON

Although Weidman (1907) mapped the geology of north-central Wisconsinand paid special attention to the mineralogy of the syenites near Wausau,Emmons and Snyder (1944) hypothesized formation of the Stettin syenitebody by metasomatism of feldspathic rocks along shear zones with alkali-rich solutions derived from a subjacent granite batholith. Turner (1948)studied the heavy accessory minerals and radioactivity of the Stettinpluton, and Geisse (1951) described the petrography of this pluton. Pet-rographic and geochemical investigation of the mafic minerals and nephelineof the Stettin pluton initiated analytical studies which have been extendedby the work of Sood and Berlin (in Sood and others, 1980)

The concentrically zoned Stettin pluton (Figure 23) is oval in plan,elongated northeasterly, with a length of 5.5 miles and a width of 4.0 miles.Older volcanic rocks enclosing the pluton have been extensively syenitized.The eastern and southern margin of the pluton is a complexly laminated seriesof altered volcanic screens and pendants and various, contaminated intru-sive phases of the syenite including nepheline syenite. The wall zonecomprises a discontinuous outer rim of gneissic nepheline syenite, andan inner layer of tabular syenite (Stop ). The intermediate zone (Stops#6 and #7) is composed of amphibole and pyroxene syenite showing consider-

?çWausau Pluton

Figure 23-— Generalized geologic map of the Stettin syenite pluton. Excerptedfrom Figure 6 in this guidebook.

THE STETTIN SYENITE PLUTON

Figure 23-- Generalized geologic map o f t he S t e t t i n syen i te p luton. Excerpted from Figure 6 i n t h i s guidebook.

Although Weidman (1907) mapped the geology of nor th-cent ra l Wisconsin and pa id specia l a t t e n t i o n t o the mineralogy of the syeni tes near Wausau, Emmons and Snyder (1944) hypothesized format ion o f the S t e t t i n syeni t e body by metasomatism of f e l d s p a t h i c rocks along shear zones w i t h a l k a l i - r i c h so lu t i ons der ived from a subjacent g r a n i t e bath01 i th . Turner (1948) s tud ied the heavy accessory minerals and r a d i o a c t i v i t y of t he S t e t t i n pluton, and Geisse (1951) described t h e petrography of t h i s p luton. Pet- rographic and geochemi ca l i n v e s t i g a t i o n of t he mafic mineral s and nephel i n e . o f the S t e t t i n p l u t o n i n i t i a t e d a n a l y t i c a l s tud ies which have been extended by the work of S00d and B e r l i n ( i n sood and others, 1980)

The c o n c e n t r i c a l l y zoned S t e t t i n p l u t o n (Figure 23 ) i s oval i n plan, elongated nor theaster ly , w i t h a l eng th of 5.5 mi les and a w id th of 4.0 mi les . 01 der vo lcan ic rocks enclos ing the p i uton have been ex tens ive l y syeni ti zed. The eastern and southern margin o f t he p l u t o n i s a complexly laminated ser ies of a l t e r e d vo lcan ic screens and pendants and various, contaminated i n t r u - s i v e phases of t he syen i te i n c l u d i n g n m h e l i n e syeni te. The w a l l zone comprises a discont inuous ou te r r i m of gne iss i c nephel ine syenite, and an inne r l a y e r o f t abu la r syen i te (Stop # ). The in termedia te zone (Stops #6 and #7) i s composed o f amphibole and pyroxene syen i te showing consider-

-30-

able variation in composition and texture. The amphibole syenite is com-monly quartz-bearing. The core zone (Stop #8) is one mile in diameterand is located asymmetrically near the north end of the pluton. The corezone comprises a well-defined, cylindrical rim of indistinctly banded neph-eline syenite surrounding a core of pyroxene syenite. Field relations in-dicate the following intrusion sequence: (1) pyroxene syenite, (2) nephelinesyenite, (3) tabular syenite, (4) amphibole syenite. Numbers 3 and 4 could bereversed. This evidence is based wholly on field relations (Myers). Itshould also be emphasized that the intrusion sequence may not be the same asthe crystallization seqence. Analytical work (Sood and Berlin, this guidebook)suggests a very late age for the nepheline syenite. (See discussion ofpetrochemistry beginning on page 31 ).

A summary tabulation of paragenetic relations of minerals in each zone ofthe Stettin syenite pluton is presented with modification from Koeliner(1974) in Figure 27, p. 36.

The section on Mineral chemistry and petrology, originally contained inthe 1980 Wausau field trip Guidebook #3 by Sood and Berlin has been excerptedunchanged and incorporated in this guidebook.

able variation i n composition and texture. The amphibole syeni t e i s commonly quartz-bearing. The core zone (Stop #8) i s one mile in diameter and i s located asymmetrically near the north end of the pluton. The core zone comprises a we1 1 -defined, cylindrical rim of indis t inct ly banded neph- e l ine syenite surrounding a core of pyroxene syenite. Field relations indicate the fol lowing intrusion sequence: (1 ) pyroxene syenite, (2 ) neohel ine syeni t e , (3) tabular syenite, (4) amphibole syenite. Numbers 3 and 4 could be reversed. This evidence i s based wholly on f i e ld relations (Myers). I t should also be emphasized that the intrusion sequence may not be the same as the crystal 1 ization seqence. Analytical work (Sood and Berlin, t h i s guidebook) suggests a very l a t e age fo r the nephel ine syenite. (See discussion of petrochemistry beginning on page 31 ).

A summary tabulation of paragenetic relations of minerals i n each zone of the S te t t in syeni t e pi uton i s presented w i t h modification from Koel lner (1974) in Figure 27, p. 36.

The section on Mineral chemistry and petrology, or iginal ly contained in the 1980 Wausau f ie ld t r i p Guidebook #3 by Sood and Berlin has been excerpted unchanged and incorporated in th i s guidebook.

—31—

MINERALOGY AND MINERAL CHEMISTRY

(STOP NO's 8 through 12)

M.K. Sood, PE. .Myers, and LA. Beii

The principal mineral phases in Stettin Complex are perthitic feldspars,nepheline, sodic and calcic pyroxenes, and sodic amphiboles whose representa-tive chemistry is given in Table and characteristics described below:

Eel dspars

The major phase of feldspar is a microperthite in uniform veins showingparallel, subparallel, or wavy lamellar intergrowths, or as patches of onefeldspar in the host. Both perthite and antiperthite are present,although perthite is more common than antiperthite. Frequently the tabularfeldspar grains exhibit Carlsbad twinning and less coninonly Mannebach twinning.The perthitic feldspar constitutes 80 to 90 percent of the syenites and 60 to75 percent of the nepheline syenites (Table 4).

Distinct grains of albite have an average extinction angle of ]50, butare not common in any of the syenites.

Microcline, also present as distinct grains, show its characteristicspindle-shaped polysynthetic twinning and wavy extinction, but is less abundantthan albite as individual grains.

The bulk compositions of the perthitic alkali feldspars were determinedfor nine samples of three major zones of the0Stettin complex. The sampleswere homogenized to a sariidine phase at 1050 in a muffle furnace for 48 hours;then 20 = 201 feldspar - 101 KBrO3 CuKcx was measured and the molecular percentorthoclase was determined using the homogenized natural microcline-low albitex-ray determinative curve of Jones etal. (1969) The compositions are givenbelow in Table 4

Table 4

THE MOLECULAR PERCENT ORTHOCLASE OF HOMOGENIZEDPERTHITIC ALKALI FELDSPARS OF THE STETTIN ROCKS

Sample 29 CuKa Mol % Or

Core Zone

pyroxene syenite 1.40° 39

Intermediate Zone

amphibole syenite3 amphibole syenites

1.451.40

3539

Rim Zone

tabular syenitenepheline syenitenepheline syenite

1.431.351.39

374441

The molecular percent orthoclase ranges from 35 to 44%; however, Or%is above 40% for the nepheline syenites and is less than 40% for the nepheline-free syenites.

MINERALOGY AND MINERAL CHEMISTRY

(STOP N O ' S 8 through 12)

M.K. Sood, P,E..Myers, and L.A..Berlirt 1

The p r i n c i p a l minera l phases i n S t e t t i n Complex a r e p e r t h i t i c f e l dspars, nepheline, sodic and c a l c i c pyroxenes, and sodic amphiboles whose representa- t i v e chemist ry i s given i n Tab1 e and c h a r a c t e r i s t i c s described below:

Fel dsoars

The major phase o f f e ldspa r i s a m i c r o p e r t h i t e i n un i fo rm veins showing p a r a l l e l , subpara l l e l , o r wavy l a m e l l a r in tergrowths, o r as patches o f one fe ldspa r i n t he host. Both p e r t h i t e and a n t i p e r t h i t e a re present, a l though p e r t h i t e i s more common than a n t i p e r t h i t e . Frequent ly the t a b u l a r f e ldspa r g ra ins e x h i b i t Carlsbad tw inn ing and l e s s commonly Mannebach twinning. The p e r t h i t i c f e ldspa r c o n s t i t u t e s 80 t o 90 percent o f t he syen i tes and 60 t o 75 percent o f t he nephel ine syeni tes (Table 4 ) .

D i s t i n c t g ra ins o f a l b i t e have an average e x t i n c t i o n angle o f 15O, b u t a r e n o t common i n any of the syeni tes.

Mic roc l ine, a l so present as d i s t i n c t gra ins, show i t s c h a r a c t e r i s t i c spindle-shaped po l ysyn the t i c tw inn ing and wavy e x t i n c t i o n , b u t i s l e s s abundant than a l b i t e as i n d i v i d u a l g ra ins .

The bu l k composit ions o f the p e r t h i t i c a1 ka l i fe ldspars were determined f o r n ine samples o f th ree major zones of t h e s t e t t i n complex. The samples were homogenized t o a san id ine phase a t 1050 i n a m u f f l e furnace f o r 48 hours; then ~ 2 0 = 201 fe ldspa r - 101 KBr03 CuKa was measured and the molecular percent o r thoc lase was determined us ing the homogenized na tu ra l mic roc l ine- low a l b i t e x- ray de terminat ive curve o f Jones -- e t a1. (1969) The composit ions a re g iven below i n Table 4

Table 4

THE MOLECULAR PERCENT ORTHOCLASE OF HOMOGENIZED PERTHITIC ALKALI FELDSPARS OF THE STETTIN ROCKS

Sample

Core Zone

pyroxene syen i te

In te rmed ia te Zone

amphi bo l e syeni t e 3 amphibole syen i tes

Rim Zone

tabu1 a r syeni t e nephel ine syen i te nephel i n e syeni t e

Mol % O r

The molecular percent o r thoc lase ranges f rom 35 t o 44%; however, O r % i s above 40% f o r t h e nephel ine syen i tes and i s l ess than 40% f o r t he nephel ine- f r e e syeni t es .

-32—

The intensity ratios of the 201 peaks of rnicrocline and albite weredetermined for the perthitic feldspars by scanning in both directions between200 and 23°-20 Cukc at l/8°-20 per minute using 200 counts per full chartscale, a time constant of 5 seconds and a chart speed of 15 inches per hour.The angular positions were averaged from three scans. Then the goniometerwas exactly centered on one peak at a time and the intensity was measuredusing a fixed time of ten seconds with a 2 second time constant. The back-ground intensity was measured at the midpoint between the two peaks.

Then: A = number of counts on microcline 2b1/l0 sB = number of counts on low albite 201/10 SC = number of counts on the background/b s

The intensity ratio 'o"a (A - C)/(B - C).

The intensity ratio and the value of the bulk composition of Or%/Ab%for each of the perthitic feldspars studied were plotted on the graph ofKuellrner (1959)(Figure24).

From this diagram, implications can be made as to the temperature-structural state of the feldspars. From the plots a broadening ratio (B)is obtained.

The broadening ratio is a measure of the distortion or structuralmistakes in the two phases of perthite. The broadening ratio will decreasewith slower crystallization and lower temperature since these conditions arefavorable for the attainment of an ordered arrangement of Si and Al ionsin the tetrahedral sites of the feldspar structure (Smith, 1974).

The broadening ratios for the perthitic alkali feldspars of the Stettinrocks range from low (B = 0.30) to intermediate values (B 0.9). This isan indication of the low temperature-structural state of these perthites,corresponding to the maximum to intermediate microcline-low albite seriesdetermined from the positions of the 204 and 060 reflections.

8— // // /,/ —

6 /. / / —5 / / /// / /, /4 / / // / —/ / // /3 / / // , —/ / / // / / /2 / / —/ /' // , // q' / / / // / . / // / b',•'• / /

o i " / / / / // / / ., / /.8 / /71 / /6 / // • //5

// /7 / , Figure 24—— Plot of bulk compositions4

//// / 7 Or%/ Ab% versus lo/lafor the 201 re-, // •• / 7 fboctions of the two feldspar phases/ ," // in the Stettin perthite for determin-..2 / / / ation of broadening ratios (B) after// ,/ Kucilmen, 1959./ /

/i ii I I

.1I 11111! I I I

.1 .2 .4 .6 .8 1 2 4 6 8 10

ORTHOCLASE %ALBITE %

The in tens i ty r a t i o s of the 701 peaks of microcline and a l b i t e were determined f o r the p e r t h i t i c feldspars by scanning i n both di rect ions between 20Â and 230-20 C u k a a t 1/8Â°-2 per minute using 200 counts per f u l l char t sca le , a time constant of 5 seconds and a char t speed of 15 inches per hour. The angular posit ions were averaged from three scans. Then the goniometer was exactly centered on one peak a t a time and the in tens i ty was measured using a fixed time of ten seconds w i t h a 2 second time constant. The background in tens i ty was measured a t the midpoint between the two peaks.

Then: A = number of counts on microcl ine 201/10 s B = number of counts on low a l b i t e 201/10 s C = number of counts on the background/IO s

The in tens i ty r a t i o I 0 / I = ( A - C)/(B - C).

The in tens i ty r a t i o and the value of the bulk composition of Or%/Ab% for each of the p e r t h i t i c feldspars studied were plot ted on the araph of Kuel lmer (1959) (Figure 24) .

From t h i s diagram, implications can be made as t o the temperature- s t ruc tura l s t a t e of the fe ldspars . From the p lo t s a broadening r a t i o ( B ) i s obtained.

The broadening r a t i o i s a measure of the d i s to r t ion o r s t ruc tura l mistakes i n the two phases of per th i te . The broadening r a t i o wil l decrease with slower c ry s t a l l i z a t i on and lower temperature s ince these conditions a r e favorable f o r the attainment of an ordered arrangement of Si and A1 ions in the te t rahedral s i t e s of the fe ldspar s t ruc ture (Smith, 1974).

The broadening r a t i o s fo r the p e r t h i t i c a1 kali feldspars of the S t e t t i n rocks range from low (B = 0.30) t o intermediate values ( B = 0.9). This i s an indication of the low temperature-structural s t a t e of these pe r th i t e s , corresponding t o the maximum t o intermediate microcline-low a l b i t e s e r i e s determined from the posit ions of the 204 and 060 re f lec t ions .

Figure 24-- Plot of bulk compositions Or%/ Ab% versus I O / I f o r the 201 re- f l ec t ions of the two feldspar phases in the S t e t t i n per th i te fo r determinat ion of broadening r a t i o s ( B ) a f t e r Kucllmen, 1959.

O R T H O C L A S E % A L B I T E %

-33-

TABLE 5

ELECTRON MICROPROBE CHEMICAL ANALYSES OF MAJOR MINERALS OF STETTIN COPPLEX

Feidsoar NnDI' bo1esPyroxene

nsy DSV tsv Nehe1ineFe-TI -

OxIdsSi02 67.42 68.23

A1203 19.23 19.72

Ti02 -- ——

FeO -- --MnO -- --MgO -- --CaO 0.44 0.25

Na20 7.40 11.23

K20 6.39 0.27

40.45

8.70

3.17

29.1

0.81

2.28

10.3

2.14

1.57

39.90

9.28

1.31

34.22

1.00

0.73

8.89

3.30

1.75

48.1

1.0

0.1

26.8

1.52

0.62

17.8

2.40

——

50.1

0.59

0.26

23.70

0.89

4.41

20.30

0.57

N.D.

50.8

1.34

0.30

26.40

0.71

0.99

11.00

6.80

N.D.

46.5

33.1

-—

0.18

----0.13

15.20

5.46

0.44

0.25

7.57

90.40

1.19

----—-

-—

ATOMIC PROPORTIONS

Fledspars Based Ainphiboles Based Pyroxene Basedon 8 oxygens on 23 oxygens on 6 oxygens

NephelineBased on

32 oxygens

Fe..TiOxides

Based on24 oxygens

Si 2.987 2.988

Al 1.004 1.018

Al 0 0

Ti -- --Fe -- --Mg -- --Ca 0.021 0.012

Na 0.635 0.954

K 0.361 0.015

Ae

Di

Hd

6.73 6.43

1.52 1.76

0.118 0

0.372 0.147

3.89 4.61

0.515 0.172

1.765 1.536

0.690 1.031

0.323 0.361

2.001 1.990 2.074

-- 0.010 --0.42 0.017 0.064

0.004 0.008 0.010

0.930 0.786 0.902

0.38 0.261 0.060

0.797 0.862 0.480

0.194 0.044 0.538

—— —— ——

16.0 3.9 47.3

3.5 25.2 5.3

75.1 60.8 42.8

8.76

7.352

----

0.29

--0.27

5.52

1.315

0.119

0.079

1.530

20.310

0.272

--------

* From Koeliner (1974)

Nephel me

Nepheline is characterized by its subhedral to euhedral, rectangular, blockyform and parallel extinction in thin section. Nephaline syenite pegmatites con-.tam nepheline crystals up to 5 cm lont. In hand specimen the nepheline ispinkish and greasy in appearance. Alteration leaves gray, etched surfaces ofnegative relief against feldspar and quartz. Cancrinite is common at STOP 8, andparagonite has been reported. Koeliner (1974) reports that the nephelines fromthe Stettin pluton are enriched in Si by about 15% and deficient in alkalies byabout 13%.

Pyroxenes

Both sodic and calcic clinopyroxenes occur in the Stettin pluton. Representativechemical compositions are given in Table 5 above. Aegiring and aegirine-augitemay or may not be rimmed by sodic amphiboles. Some are color zoned with brightgreen rims and pale yellowish green cores. Average extinction angles of pyroxenecores is 28°, whereas that of the rims is 13 to 24°. This suggests an outwardincrease in aegiring content. Calcic pyroxenes (diopside—hedenbergite) are iron-rich with aegirine content up to 10% (Koeliner, 1974). In general, Na + Fe3 ishighest in the wall zone. See photomicrograph, Figure 24.

TABLE 5

ELECTRON MICROPROBE CHEMICAL ANALYSES OF MAJOR MINERALS OF STETTIN

I Ã I Pyroxene Fpldinar SiO, 67.42 68.23

A1203 19.23 19.72

T i O2 - - -- FeO -- - - MnO -- -- MOO - - -- CaO 0.44 0.25

Na20 7.40 11.23

K2Â 6.39 0.27

Fledspars Based on 8 oxygens

S i 2.987 2.988

ATOMIC PROPORTIONS

Amphiboles Based on 23 oxygens

6.73 6.43

1.52 1.76

0.118 0

0.372 0.147

3.89 4.61

0.515 0.172

1.765 1.536

0.690 1.031

0.323 0.361

DSV 50.1

0.59

0.26

23.70

0.89

4.41

20.30

0.57

N.O.

50.8 1.34

0.30

26.40

0.71

0.99

11 .oo 6.80

N.D.

* Pyroxene Based on 6 oxygens

2.001 1.990 2.074 -- 0.010 --

0.42 0.017 0.064

0.004 0.008 0.010

0.930 0.786 0.902

0.38 0.261 0.060

0.797 0.862 0.480

0.194 0.044 0.538 - - -- - -

16.0 3.9 47.3

3.5 25.2 5.3

75.1 60.8 42.8

DUPLEX

~ e o h e l ine* 46.5

33.1 -- 0.18 -- - - 0.13

15.20

5.46

* {epheline Based on

32 oxygens

8.76

7.352 -- - -

0.29 --

0.27

5.52

1.315

Fe-TI Oxides 0.44

0.25

7.57

90.40

1 . I9 -- -- -- - -

Fe-Ti Oxides

Based on 24 oxygens

0.119

0.079

1.530

20.310

0.272 -- -- -- --

- * From Koellner (1974)

Nephel i ne

Nephel i n e i s cha rac te r i zed by i t s subhedral t o euhedral , rec tangu la r , b l ocky form and p a r a l l e l e x t i n c t i o n i n t h i n sec t ion . Nephal ine s y e n i t e pegmati tes con- t a i n nephe l ine c r y s t a l s up t o 5 cm l e n t . I n hand specimen t h e nephe l ine i s p i n k i s h and greasy i n appearance. A l t e r a t i o n leaves gray, e tched sur faces o f nega t i ve re1 i e f aga ins t f e l d s p a r and quar tz . paragoni t e has been repor ted . Koe l l n e r (1 974) t h e S t e t t i n p l u t o n a re enr i ched i n S i by about about 13%.

a n c r i n i t e i s common a t STOP 8, and r e p o r t s t h a t t h e nephel ines from 15% and d e f i c i e n t i n a1 ka l i e s by

n t h e S t e t t i n p lu ton . Representat ive

Pyroxenes

Both sod i c and c a l c i c c l inopyroxenes occur i chemical composit ions a re g i v e n i n Tab1 e 5 above. A e g i r i n g and aeg i r i ne -aug i t e may o r may n o t be rimmed by sod i c amphiboles. Some a re c o l o r zoned w i t h b r i g h t green r ims and pa le y e l l o w i s h green cores. Average e x t i n c t i o n angles o f pyroxene cores i s 28O, whereas t h a t o f t h e r ims i s 13 t o 24'. Th i s suggests an outward inc rease i n aegi r i n g content . Cal c i c pyroxenes (d iops ide-hedenbergi t e ) a r e i r o n - r i c h w i t h a e g i r i n e con ten t up t o 10% (Koe l l ne r , 1974). I n general , Na + ~ e ^ + i s h i ghes t i n t h e w a l l zone. See photomicrograph, F igure 24.

Figure 25—- Photomircograph of pyroxene syenite. Zirconcrystals (left of center) surrounded by dark arfvedson—ite and aegirine-augite. Mafic cluster enclosed instained alkali feldspar. Plane polarized light.

Amphibol es

The dominant mafic mineral is a bluish green sodic amphibole with an absorptionscheme closely resembling arfvedsonite: X = bluish green or greenish blue; Z =greenish brown or light brown. Average extinction angle is 16°. Late blueamphibole is riebeckite with absorption sheme: X = deep blue; Z = light blue.Extinction angle of these grains is very low. X-ray diffraction powder patternsof the riebeckite give 8.42A for the ll0] reflection compared to 8.50A forarfvedsonite [110] reflections. All of the amphiboles are iron—rich.Biotite

Two varieties of biotite occur sparsely in the Stettin pluton: dark orange—brown, and light olive green to brown. This indicates two genetic associationsof biotite with bimodal partitioning of Ti, Fe2, Fe3, and Mn2. Electron micro—probe analysis of biotite from the Stettin pluton by Myers shows the orange-brownvarieties to be Ti and Fei+ rich.Accessory Minerals

Numerous accessory minerals have been identified. They include zircon, fluor-apatite, fayalite, magnetite, sphene, fluorite, calcite, cancrinite, allanite,pyrochlore, and many others.

Modal compositions of Stettin rocks are included in Table 6. (from Koeliner, 1974).

—34-

F igure 25-- Photomircograph o f pyroxene syen i t e . Z i r con c r y s t a l s (1 e f t o f c e n t e r ) surrounded by dark ar fvedson- i t e and aeg i r i ne -aug i t e . M a f i c c l u s t e r enc losed i n s t a i n e d a1 k a l i fe ldspar . Plane p o l a r i z e d 1 i g h t .

Amphi bo l es

The dominant m a f i c m inera l i s a b l u i s h green sod i c amphibole w i t h an abso rp t i on scheme c l o s e l y resembl ing a r f v e d s o n i t e : X = b l u i s h green o r g reen ish b lue ; Z = g reen ish brown o r l i g h t brown. Average e x t i n c t i o n ang le i s 16O. La te b l u e amphibole i s r i e b e c k i t e w i t h abso rp t i on sheme: X = deep b lue ; Z = 1 i g h t b lue. E x t i n c t i o n ang le o f these g r a i n s i s v e r y low. X-ray d i f f r a c t i o n powder p a t t e r n s o f t h e r i e b e c k i t e g i v e 8.42A f o r t h e [110] r e f l e c t i o n compared t o 8.50A f o r a r f vedson i t e [I 101 r e f 1 ec t i ons . A1 1 o f t h e amphi bo l es a re i r o n - r i c h .

B i o t i t e

Two v a r i e t i e s o f b i o t i t e occur spa rse l y i n t h e S t e t t i n p l u t o n : dark orange- brown, and l i g h t o l i v e green t o brown. Th i s i n d i c a t e s two gene t i c assoc ia t i ons o f b i o t i t e w i t h bimodal p a r t i t i o n i n g o f T i , ~ e * + , ~ e 3 + , and ~n2'. E l e c t r o n mic ro - probe a n a l y s i s o f b i o t i t e s f rom t h e S t e t t i n p l u ton by Myers shows t h e orange-brown v a r i e t i e s t o be T i and r i c h .

Accessorv M inera l s

Numerous accessory m i n e r a l s have been i d e n t i f i e d . They i n c l u d e z i r c o n , f l uor - a p a t i t e , f a y a l i t e , magnet i te , sphene, f l u o r i t e , c a l c i t e , c a n c r i n i t e , a1 l a n i t e , py roch l ore, and many o the rs .

Modal composi t ions o f S t e t t i n rocks a r e i n c l u d e d i n Table 6 . ( f r om Koe l l ne r , 1974).

—35-

TABLE 6?43DAL COMPOSITIONS OF THE STETTIN ROCKS

INTERMEDIATE ZONE

from Sood, Myers, and Berlin, 1980, p. 28.

CORE ZONE WALL ZONE

Qel *JI,vh,m

Qet Tin

Unt,ni.rmiigr Granit,

pay Pyranin, Sp,nIe

say *mpliib.i. Sy.ait.apap S,niI. aplit.

GXPLANAIION

tay 1.b.,i., Sp.iji.nay NiphnIia• Syenit*

I lay L.n..Idi y.niI.I ayv Spanitiand V,lc.niu

nvb Irntjafed Maftc Veltanksiv i.lik V.k..,na

U.n lulit islienka

Figure 26-.. Geologic map of the Stettin complex (after Myers. 1973)

including localities of samples and field trip stops.

ROCK TYPE Amphibole Syenite PyroxeneSyenite

TabularSyenite

Nepheline Syenite

AMPLE NUMBERS* 10 77 503 100 6 and 504 65 46 2 92

uartz 7.1 6.6 2.9 1.4

ephellne 26.4 17.6 6.6Perthite 80.7 83.5 90.3 83.0 87.4 80.2 63.6 75.7 61.4

lblte 0.5 0.2

mphibole 11.2 8.6 5.1 13.6 5.5 19.1 8.4 4.6 29.5

Pyroxene 0.6 4.1

Blotite' 0.2 0.5 0.2 0.6 0.4 0.4

Biotite (alter.) 0.6 0.3 0.5

Zircon 0.2 0.2 0.1 0.7

patite 0.1

Fluorite 0.2 0.5

Calcite 0.3 0.1

Sphene 1.1

Opaque minerals 0.1 0.1 0.3 0.2 1.3 0.1 1.0 0.5

AlteratIon 0.4 0.2 0.3 0.3 0.4 0.5

0

I

Calci te

Sphene

Opaque minerals

TABLE 6 MODAL COMPOSITIONS OF THE STETTIN ROCKS

INTERMEDIATE ZONE

Amphibole Syenite

CORE ZONE

Pyroxene Syeni t e

6 and 504

- Tabular Syeni t e

MALL ZONE

iepheline Syeni t e

from Sood, Myers, and Berlin, 1980, p. 2 8 .

Figure 26% Geologic map of the S te t t i n complex ( a f t e r Myers, 1973) including l o c a l i t i e s o f samples and f i e l d t r i p stops.

l t y Itbultr Sltiiite

nÃ§ Mtphtiint Sytnitt 4 I i y LÃ§mulda Sytniit Syv Sltnifi~Ã§ Volconiti

Â mvb Int t ia t td Malit Vsltanio

fv Feliit hlimniti

mv Mrnlli vÃ§itrnk

LU

N1

LU

H-

LU

LU

H-

FIGURE 27

OF MINERALS IN EACH ZONE OF THE STETTIN PLUTON

—36-

PARAGENETIC RELATIONS

ROCK TYPE

TABULAR SYENITE(Myers, 1973)

CRYSTALLIZATION

LU

-J

SEQUENCE

— zircon —I

I —pyroxene —1_.alka1i feldspar—

f—opaques_._

I green amphiboles— I

1_biotite__I

NEPHELINE SYENITE(Koellner, 1974,

—nepheline -I

I—alkali feldspar—II—olivine—I

I—pyroxene---

I- opaques—l

—green amphiboles.—4

Jbiotite—4

PYROXENE SYENITE(Koellner, 1974,p. 12)

1.-alkali feldspar—IEapatite-i

— opaques—I

I— olivine————.-$

I— pyroxene—

— green amphibole—S

i—biotite—..

frcarbonate.-4

i—blue amphibole—

AMPHIBOLE SYENITE(Koeliner, 1974,p. 33)

I—alkali feldspar—IIapatite4

I—opaques—4

I—pyroxene —I

)—green amphibole—I

I—biotite—4

I—blue amphibole—

From Sood, Myers, and Berlin, 1980, p. 24

FIGURE 27 PARAGENETIC RELATIONS OF MINERALS IN EACH ZONE OF THE STETTIN PLUTON

- - -

R O C K T Y P E

TABULAR SYENITE (Myers, 1973)

NEPHELINE SYENITE ( K o e l l n e r , 1974, P. 1 2 )

PYROXENE SYENITE (Koel 1 n e r , 1974, P. 1 2 )

AMPHIBOLE SYENITE ( K o e l l n e r , 1974, P. 33)

C R Y S T A L L I Z A T I O N S E Q U E N C E - - - - - -

- z i rcon 4

!-pyroxene --4 j ~ a l ka l i f e l d s p a r 4

IÃ‘opaques 1- green amphi bol es--4

k b i o t i t e+

-nephel i ne '-1

k a l ka l i f e l d s p a r 4

l - o l i v i n e 4

+pyroxene j

1- opaquesÃ‘

g reen amphi bol esÃ‘-

Ã ˆ - b i o t te-Ã

I- a1 kal i f e l d s p a r -I

I- a p a t i te-i - o p a q u e s - l

01 iv ine-- - -4

+ pyroxene-

+ green amphi boleÃ‘

t- b i o t i t e 4

p c a r b o n a t e - t

I- bl ue amphi bol e-

l- a1 kal i f e l d s p a r - l

k a p a t i te-1 k o p a q u e s l

+ p y r o x e n e 4

+green amphi bol e Ã ‘

t Ã ‘ b i 0 t i t e -

IÃ‘b ue amphi bol e -

From Sood, Myers, and Berl i n 1980, p. 24

—37—

STOP #8

TITLE: Nepheline syenite and syenitized volcanic rocks, Stettin pluton wall zoneLOCATION: County Highway U on hill crest 0.5 mile east of Little Rib River;

SW , SW , Sec. 18, T29N, R7E; Wausau 15' quadrangle. (See Fig. 6).AUTHOR: Paul E. Myers, University of Wisconsin — Eau ClaireDATE: April, 1984


Intrusion of the Stettin pluton was accomplished by peeling slices away fromthe cylindrical walls and carrying them upward while causing much metasomaticalteration through the addition of alkalies and alumina. The border phasesand alterations typical of the eastern border of the Stettin pluton are wellexposed along the highway at this location.DESCRIPTION:

The dominant rock types at this location are: (1) pyroxene, amphibole, andolivine—bearing nepheline syenite pegmatite, and variously syenitized maficvolcanic rocks. Many hybrid types are present also. Through addition of K, Na,and Al the volcanic rocks were converted locally to aplitic rocks composed ofsodic amphibole(s), biotite, and alkali feldspar (perthite). The altered vol-canic rocks have a characteristic splotchy or streaked appearance owing to localvariation in mineral proportions: fine-grained K-feldspar imparts a pinkish colorto the altered portions of the volcanic rocks, and primary pyroxenes are alteredto Na-Fe pyroxenes.

ns = nepheline syenite

Figure 28—— Geologic strip map along County Hiqhway U.

The nepheline syenite pegmatite at Location A (Figure 28) is composed of verycoarse subhedral grains of ribbon perthite and 1 to 2 centimeter subhedral crystalsof pinkish, altered nepheline (10—20%). Dark mineral is mainly dark green toyellow green amphibole. Accessory minerals are zircon, apatite, and monazite,

The mafic volcanic rocks at Location B are composed of plagiocalse, perthiticalkali feldspar, green amphibole, brown biotite, and anhedral epidote. It islocally cut by syenite veinlets.

Syenitized volcanic rocks at C have dark areas of residual mafic rock nowcomposed of anhedral green amphibole (X = dark yellow green; Z = dark olive greento brown) clustered with subhedral blue—gray amphibole (riebeckite) and clots ofpale yellow—green epidote. Brown biotite occurs in coarser varieties.

C B A—EAST--'-

my

490' 390' 2p0' 100'

sv = syenitized volconics s = syenite

mv= mof Ic volcanic rocks

9'

in p1 uton wall zone

STOP #8

TITLE: Nepheline syenite and syenitized volcanic rocksy S te t t - LOCATION: County Highway U on h i l l c res t 0.5 mile east of L i t t l e

SW , SW , Sec. 18, T29N, R 7 E ; Wausau 15' quadrangle.

AUTHOR: Paul E . Myers, University of Wisconsin - Eau Claire

DATE -a April, 1984

SUMMARY OF FFATURFL:

Rib River; (See Fig. 6 ) .

Intrusion of the S te t t in pluton was accomplished by peeling s l i ces away from the cylindrical walls and carrying them upward while causing much metasomatic al terat ion through the addition of a1 kalies and alumina. The border phases and al terat ions typical of the eastern border of the S te t t in pluton are well exposed along the highway a t t h i s location.

DESCRIPTION:

The dominant rock types a t t h i s location are: (1) pyroxene, amphiboley and olivine-bearing nepheline syenite pegmatite, and variously syenitized mafic volcanic rocks. Many hybrid types are present also. Through addition of K y Na, a n d A1 the volcanic rocks were converted locally to a p l i t i c rocks composed of sodic amphibole(s), b io t i t e$ a n d a1 ka1 i feldspar ( ~ e r t h i t e ) ~ The a1 tered volcanic rocks have a character is t ic splotchy or streaked appearance owing t o local variation in mineral proportions: fine-grained K-feldspar imparts a pinkish color t o the altered portions of the volcanic rocksy and primary pyroxenes are altered t o Na-Fe pyroxenes.

sv = syenitized volcanics s = syenite ns = nepheline syenite mv= mafic volcanic rocks

Figure 28-- Geologic s t r i p map along C o u n t y Hiqhway U.

The nephel ine syenite pegmatite a t Location A ( ~ i g u r e 28) i s composed of very coarse subhedral grains of ribbon perthite a n d 1 to 2 centimeter subhedral crystals of pinkish$ altered nepheline (10-20%). Dark mineral i s mainly dark green t o yellow green amphibole. Accessory minerals are zircon, apa t i te$ and monazite-

The mafic volcanic rocks a t Location B are composed of plagiocalse, per thi t ic a1 ka1 i feldspary green amphiboley brown b io t i t ey and anhedral epidote. I t i s locally cut by syenite veinlets.

Syenitized volcanic rocks a t C have dark areas of residua1 mafic rock now composed of anhedral green amphibole ( X = dark yellow green; Z = dark 01 ive green to brown) c1 ustered with subhedral ti1 ue-gray amphi bo1 e (riebecki t e ) and c lo ts of pale yellow-green epidote. Brown bio t i te occurs in coarser var iet ies .

TITLE:

-38-

STOP #9

Contact relations and minerals in the Wall Zone, Stettin syenitepluton

LOCATION: County HighwaySE¼, SE¼, Sec.Location 92)

O at 10146 Stettin Road, Paul Knopp property,22, T29N, R6E, Marathon 15' quadrangle, (Sample

AUTHORS: P.E. Myers and M.KSbod

DATE: February 1973, February 1980


The outermost rim of the Stettin pluton is gneissic nepheline syenitecomposed mainly of alkali feldspar, perthite, nepheline, aegirine, sodicamphibole and biotite. It is in sharp contact with, and veined by, tabularsyenite composed of coarse, well-oriented laths of perthite, sodic amphi-bole, pyroxene, and lensoidal mafic inclusions composed essentially of thesame minerals but in different porportions and of finer grain size. Themafic inclusions are well-oriented parallel to the tabular fabric of theenclosing syenite and to the wall of the pluton. They contain large perth-ite porphyroblasts of similar composition and size as those in the enclosingsyenite. Zircons were mined at this site in the 1950's. Zircons from thissite have given a U/Pb age of 1520 + 20 m.y. by W.R. Van Schmus (oral com-munication).

The chief questions to be answered at this site are: (1) how were thenepheline syenite and tabular syenite emplaced, and (2) to what extent isthe present mineral assemblage a result of metasomatic replacement?

STOP #9

TITLE: Contact relations and minerals in the Wall Zone, S te t t in syenite pl uton

LOCATION: County Highway 0 a t 10146 Ste t t in Road, Paul Knopp property, SEkY SE%, Sec. 22, T29N, R6E, Marathon 15' quadrangley (Sample Location 92)

AUTHORS: P . E . Myers and M.KeSi50d

DATE : February 1973, February 1980


The outermost rim of the S te t t in pluton i s gneissic nepheline syenite composed mainly of a1 ka1 i feldspar, perthi t e , nephel iney aegirine, sodic amphibole and b io t i t e . I t i s in sharp contact with, and veined by, tabular syeni t e composed of coarse, we1 1 -oriented la ths of perthi t e y sodic amphi - bole, pyroxene, and lensoidal mafic inclusions composed essent ial ly of the same minerals b u t in different porportions and of f ine r grain s ize. The mafic inclusions are we11 -oriented parallel to the tabular fabric of the enclosing syenite and to the wall of the pluton. They contain large perth- i t e porphyroblasts of similar composition and s ize as those i n the enclosing +

syenite. Zircons were mined a t t h i s s i t e in the 1950's. Zircons from th i s s i t e have given a U/Pb age of 1520 20 m.y. by W . R . Van Schmus (oral communication).

The chief questions to be answered a t t h i s s i t e are: (1) how were the nepheline syenite and tabular syenite emplaced, and (2) to what extent i s the present mineral assemblage a resul t of metasomatic replacement?

—39-

The abundance of zircon and hastingsite amphibole, biotite and car-bonate indicates a miaskitic trend for the nepheline and pyroxene syenites.The compositions of the nepheline and pyroxene syenites are very similar(Table 6). According to Koeliner (1974, p. 144) the amphibole syenite isagpiatic and could contain a carbonatite body.

DESCRIPTION:

The nepheline syenite (Figure 30, Tables 6 and 7) is a gray, bandedrock composed here of perthitic feldspar nepheline, olivine, pyroxene,magnetite, amphibole, and biotite. Contorted aplitic and pegmatitic bandslie roughly parallel to the wall of the pluton about 1500 feet south of here.The nepheline occurs as blocky, pinkish grains which weather much morereadily than the associated minerals, giving the rock a characteristicpitted appearance. Nepheline is partially altered to cancrinite and ironoxides. Banding, and mafic content of the nepheline syenite increase out-ward toward its contact with syenitized mafic volcanics which tren west-northwesterly. In addition to the essential minerals listed above, commonaccessory minerals include zircon and sphene of unusually large size andabundance, apatite, fluorite, allanite, sodalite, pyrochlore and thoro-gummite(?). U/Pb dating of the zircons from this site by S. Goldich (oralcommunication) gave a minimum age of 1400 m.y. More recent analyses ofthese zircons by W.R. Van Schmus yielded a U/Pb age of 1520 + 10 m.y.Thus, the Stettin syenite is about 20 million years older than the WolfRiver Batholith (oral communication).

The gneissosity and isoclinal folding exhibited by the gneissic neph-eline syenite of the wall zone on the south side of the Stettin plutonsuggest considerable differential movement of material along its outerwall. The extent to which metasomatism was involved during and after em-placement is not known. However, metasomatism was extensive, and that thenepheline syenite may consist in large part of metasomatized wall rocks.

Zircon from this locality is deep red-brown, doubly terminated euhed-ral prisms up to 14 mm in length. Some crystals display geniculate twin-ning similar to that of rutile. Chemical analyses of three zircons froma nearby site (NW¼ of Sec. 22) by F.B. Hall (in Weidman, 1907, p. 313)indicates an A1203 content of between 4.28 and 7.80 percent and an Fe903content between 1.21 and 4.47 percent. Ca, Ti, Th and rare earths we'esought but not detected.

Brown pyrochiore octahedra up to 2 mm in diameter were found at thislocation by Weidman (1907, p. 308-309).

Allanite is confined mainly to petmatitic portions in the nephelinesyenite.

Apatite and sphene of unusually large size show affinity for clustersof mafic minerals in the nepheline syenite. Large sphene crystals up to7 mm in length can be collected from nepheline syenite lenses and massesnear its contact with tabular syenite.

The compositions of the nepheline and pyroxene (Table 6) . According to Koellner (1974Â p. 144 agpiatic and could contain a carbonatite body.

The abundance of zircon and hastingsite amphibole, b io t i t e and car- bonate indicates a miaskitic trend for the ne~he l ine and pyroxene syenites.

syeni tes a re very similar )

DESCRIPTION:

the amphibole syeni t e i s

and 7) i s a grayy banded ne9 ol ivineÂ pyroxeney

The nephel ine syenite (Figure 303 Tables 6 rock composed here of per th i t ic feldspar nephel i magnetiteÂ amphibole, and b io t i t e . Contorted a p l i t i c and pegmatitic bands 1 i e roughly parallel t o the wall of the pluton about 1500 f e e t south of here. The nepheline occurs as b10cky~ pinkish grains which weather much more readily than the associated minerals9 giving the rock a character is t ic pi t ted appearance. Nepheline i s par t ia l ly al tered to cancrinite and iron oxides. Banding9 and mafic content of the nepheline syenite increase outward toward i t s contact w i t h syenitized mafic volcanics which tren west- northwesterly. In addition to the essential minerals l i s t ed abovey common accessory minerals include zircon and sphene of unusually large s ize and abundancey apa t i te , f l uori t e , a1 lani t e 9 soda1 i t e , pyrochlore and thoro- gummite(?). U/Pb dating of the zircons from t h i s s i t e by S. Goldich (oral comunication) gave a minimum age of 1400 m.y. More recent analyses of these zircons by W.R. Van Schmus yielded a U/Pb age of 1520 + 10 m.y. Thus9 the S te t t in syenite i s about 20 million years older t h z n the Wolf River Bath01 i t h (oral communication).

The gneissosity and isoclinal folding exhibited by the gneissic neph- e l ine syenite of the wall zone on the south side of the S te t t in pluton suggest considerable different ial movement of material along i t s outer wall. The extent to which metasomatism was involved during and a f t e r emplacement is not known. However9 metasomatism was extensivey and tha t the nepheline syenite may consist in large part of metasomatized wall rocks.

Zircon from th i s loca l i ty i s deep red-browny doubly terminated euhedral prisms u p to 14 rnm in length. Some crystals display geniculate twinning similar to tha t of r u t i l e . Chemical analyses of three zircons from a nearby s i t e (NWg of Sec. 22) by F.B. Hall ( i n Weidmany 1907Â p. 313) indicates an A1203 content of between 4.28 and 7.80 percent and an Fe O3 content between 1.21 and 4.47 percent. Cay Ti Th and rare earths wege sought but not detected.

Brown pyrochlore octahedra u p to 2 mm in diameter were found a t t h i s location by Weidman (1907, p. 308-309).

Allanite i s confined mainly to petmatitic portions i n the nepheline syen i t e .

Apatite and sphene of unusually large s ize show a f f in i ty f o r c lusters of maf i c mineral s in the nephel ine syeni te . Large sphene crystal s up to 7 mm in length can be collected from nepheline syenite lenses and masses near i t s contact with tabular syenite.

-40-

The tabular syenite (Figure 29, Tables 6 & 7) is composed dominantlyof coarse laths of microperthite. Vein and patch type perthites predom-inate. Poikilitic amphibole (hastingsite) rims pyroxene (intermediate be-tween acmite and hedenbergite according to Koeliner (1974, p. 65). Thetabular fabric (Figure 29) is characterized by a random orientation ofperthitic feldspar tablets in a plane parallel to the outer wall of thepluton and parallel to the long dimensions of mafic inclusions. Perthiticfeldspar tablets within mafic inclusions and across their contacts areidentical to those in the enclosing tabular syenite. The inescapable con-clusions is that the perthitic feldspar is at least partly of metasomaticorigin. Veins of tabular syenite locally cut the nepheline syenite gneissin the old quarry face at this location. Mafic inclusions comprise from5 to 80 percent of the tabular syenite. As the volume of mafic inclusionsincreases, the mafic minerals, mianly sodic amphibole, become coarselypoikilitic. Individual amphibole grains up to 12 centimeters long wereobserved in a small roadside excavation 1.5 miles east-southeast of here.Although the mafic inclusions contain a much higher percentabe of pyroxeneand olivine than the enclosing tabular syenite, they are of about the samechemical composition.

The tabular syenite forms the outermost layer on the north and westsides of the Stettin pluton where the nepheline syenite is absent. Theabundance of mafic inclusions increases outward in the tabular syenite,suggesting considerable contamination by the basaltic wallrock. A unitmapped as lensoidal syenite and a closely associated syenite aplite (Myers,1973) are found locally where the nepheline syenite is absent. The lensoid-al syenite is an aplitic, gneissose rock consisting of mafic inclusionsrich in biotite enclosed in an aplitic syenite. The syenite aplite issimilar in texture and mineral composition but relatively free of maficinclusions.

Figure 29 Typical fabric of tabular syenite showing coarsetablets of microperthite in random orientation parallel to thewall of the pluton. Microperthite laths in the lensoidal maficinclusions tend to have a preferred orientation parallel tothose in the enclosing syenite. Some of the laths crystallizedacross the edges of inclusions, thus indicating a metasorriaticorigin of at least part of the microperthite.

The tabular syenite (Figure 2 9 , Tables 6 ii 7 ) is composed dominantly of coarse la ths of microperthite. Vein and patch type perthi tes predom- inate. Poi ki1 i t i c amphi bole (hastingsi t e ) rims pyroxene (intermediate between acmi t e and hedenbergi t e according t o Koe1 lner (1974, p. 65). The tabular fabric (Figure 29) i s characterized by a random orientation of per th i t ic feldspar tablets in a plane parallel t o the outer wall of the pluton and parallel t o the long dimensions of mafic inclusions. PerthTtic feldspar tablets within mafic inclusions and across the i r contacts are identical t o those i n the enclosing tabular syenite. The inescapable con- clusions is tha t the per th i t ic feldspar i s a t leas t par t ly of metasomatic origin. Veins of tabular syenite local ly cut the nepheline syenite gneiss in the old quarry face a t t h i s location. Mafic inclusions comprise from 5 to 80 percent of the tabular syenite. As the volume of mafic inclusions increases, the mafic minerals, mianly sodic amphiboley become coarsely poi ki1 i t i c . Individual amphibole grains up to 12 centimeters long were observed in a small roadside excavation 1.5 miles east-southeast of here. Although the mafic inclusions contain a much higher percentabe of pyroxene and olivine than the enclosing tabular syenite, they are of about the same chemi ca1 composi t ion.

The tabular syenite forms the outermost layer on the north and west sides of the S te t t in pluton where the nepheline syenite i s absent. The abundance of mafic inclusions increases outward i n the tabular syenite, suggesting considerable contamination by the basal t ic wallrock. A u n i t mpped as lensoidal syenite and a closely associated syenite ap l i t e (Myers, 1973) are found locally where the nepheline syenite i s absent. The lensoid- a1 syenite i s an a p l i t i c y gneissose rock consisting of mafic inclusions rich in b io t i t e enclosed i n an a p l i t i c syenite. The syenite a p l i t e i s similar i n texture and mineral composition b u t re lat ively f ree of mafic inclusions.

Figure 29 Typical fabric of tabular syenite showing coarse tablets of microperthite in random orientation parallel to the wall o f the pluton. Microperthite laths in the lensoidal mafic inclusions tend to have a preferred orientation parallel to those in the enclosing syenite. Some o f the laths crystallized across the edges o f inclusions, thus indicating a metasomatic origin of at least part of the microperthite.

-41-

Figure 30—— Photomicrograph of nepheline syenite showingeuhedral, rectangular nepheline surrounded by albite andgreen amphibole. Crossed po1ars

Figure 31—— Photomicrograph of tabular syenite showing para-llel alignment of perthitic feldspar laths or tablets.Crossed polars.

Figure 30-- Photomicrograph of nepheline syenite showing euhedral , rectangular nephel ine surrounded by a1 bi te and green amphi bol e . Crossed pol ars

Figure 31-- Photomicrograph of tabular syenite showing para- l l e l a1 ignment of per thi t ic feldspar la ths or tablets . Crossed pol ars.

AUTHOR:

DATE:

DESCRIPTION:

Coarsely crystallized syenite pegmatite dikes containing perthitic feldsparcrystals up to 35 centimeters long cut dark brownish gray amphibole-pyroxenesyenite in this weathered roadside outcrop. The moonstone" is characterizedby a soft irridescence and brownish sheen imparted by close—spaced ribbonperthite lamellae. Interstitial poikilitic dark green to bluish green amphiboleand dark greenish brown to green pyroxene occur both in the pegmatite dikesand enclosing syenite.

A large stone quarry in the woods just north of here was operated sporadicallyuntil about 20 years ago by Mr. Gilbert Schultz who is, incidentally, disturbedwhen rockhounds invade his quarry without permission. he quarry has vertical rockwalls and is filled with deep water - DANGEROUS! The syenite in this quarry ispredominantly massive, coarse—grained, medium brownish gray syenite composed ofperthite laths with poikilitic amphibole and very subordinate green pyroxene.Pegmatite dikes exposed in the Schultz Quarry are nearly vertical, reach 0.7 mwidth, and show a distinctive "comb structure" (Figure 32). Interstitial amphi-.boles are coarser in the dikes, and tend to be concentrated along dike margins.The dikes appear to have crystallized into opening fissures under conditions ofextension. Several large, fresh boulders of a very mafic dike rock - lamprophyre(?) wer observed in the quarry, but their relationship to the syenite is unknown.

TITLE:

LOCATION:

-42-

STOP #10

"Moonstone" dikes in pyroxene—amphibole syenite of Intermediate Zoneof the Stettin syenite pluton.North side of County Highway U, 0.7 miles east of County Highway 0;SW, SE4, Sec. 28, T29N, R6E; Marathon 15' quadrangle [See Figure 6]

— &II 1) S F fl -

April, 1984

/ —

'I\i'

I"

0 6 inches ' .

/ Figure 32—— "Comb" structure in/ ' , syenite pegmatite dike cutting

,' amphibole-pyroxene syenite in'' /- the Schultz Quarry 0.2 milesnorth of here. Amphibole is

— poikilitic, and occurs with green/ '... pyroxene.1

—I

'\ /I] \ —

-42-

STOP #10

TITLE: "Moonstone" dikes in pyroxene-amphibole syenite of Intermediate Zone of the Stettim syenite pluton.

LOCATION: North s ide of County Highway U , 0.7 miles east of County Highway 0; SW:, SE?, Sec. 28, T29N, R6E; Marathon 15' quadrangle [See Figure 6 1

AUTHOR:

DATE : - Paul E. Myers, University of Wisconsin - Eau Claire

April , 1984

DESCRIPTION :

Coarsely crystall ized syenite pegmatite dikes containing per th i t ic feldspar crystal s u p t o 35 centimeters long cut dark brownish gray amphi bol e-pyroxene syenite in t h i s weathered roadside outcrop. The "moonstone" i s characterized by a sof t irridescence and brownish sheen imparted by close-spaced ribbon perthite lamellae. In t e r s t i t i a l poikil i t i c dark green to bluish green amphibole and dark greenish brown to green pyroxene occur b o t h in the pegmatite dikes and enclosing syenite.

A large stone quarry in the woods just north of here was operated sporadically until about 20 years ago by Mr. Gilbert Schul t z who i s , incidentally, disturbed when rockhounds invade his quarry without permission. he quarry has vertical rock walls and i s f i l l e d with deep water - DANGEROUS! The syenite in t h i s quarry i s predominantly massive, coarse-grained, medium brownish gray syenite composed of perthite la ths with poik i l i t ic amphibole and very subordinate green pyroxene. Pegmatite dikes exposed in the Schul t z Quarry are nearly vertical , reach 0.7 m width, and show a dis t inct ive "comb structure" (Figure 32). In t e r s t i t i a l amphiboles are coarser in the dikes, and tend to be concentrated along dike margins. The dikes appear to have crystall ized into opening fissures under conditions of extension. Several 1 arge, fresh boulders of a very mafic dike rock - lamprophyre ( ? ) wer observed in the quarry, b u t t he i r relationship to the syenite i s unknown.

Figure 32-- "Comb" s tructure in syenite pegmatite dike cutting amphibole-pyroxene syenite in the Schul t z Quarry 0.2 miles .north of here. Amphibole i s po ik i l i t i c , and occurs with green pyroxene.

-43-

STOP #11

TITLE: Amphibole and Pyroxene Syenites in the Intermediate Zone

LOCATION: NW , SW , Sec. 14, T29N, R6E; Marathon 15' quadrangle: See Fig. 6.

AUTHOR: Paul E. Myers, University of Wisconsin — Eau ClaireDATE: April, 1984SUMMARY OF FEATURES:

Massive and flow—lineated,gray to pinkish orange amphibole—pyroxene syeniteof the Intermediate Zone is well exposed on both sides of the road at thislocality. This rock closely resembles the pyroxene—bearing syenite describedon page 42. The syenites just north and west of here contain little or nopyroxene, and display complex flow structures resembling those at the OldTechnical Institute (Stop #5). The amphibole syenite north of here containsabundant riebeckite. Although the dominant mafice mineral in the pyroxene-bearing syenite here is amphibole, the pyroxene occurs as grains rimmed byamphibole. Quartz-bearing aplites are common in amphibole syenites they werenot observed in the pyroxene-bearirg syenites.DESCRIPTION:

Whereas the amphibole is characteristically pink in outcrop, thepyroxene syenite is a moderate-to-light olive gray with islands ofcoarse mafics enclosed in coarse tablets of randomly oriented feld-spar. The amphibole syenite shows considerably greater textural var-iation, even at mesoscopic scale. Although vein-like and irregularmasses of zoned pegmatite and aplite are common in all outcrops, thedominant rock type is medium-grained amphibole syenite with a faintto conspicuous lamination, with or without lineation created by align-ment of feldspar tablets and lensoidal clots of mafic minerals--main-ly amphibole and subordinate pyroxene. Pegmatitic phases of the am-phibole syenite contain up to 12% quartz as coarse segregations common-ly rimmed by blue (riebeckitic) amphibole.

In thin section, mafics are clustered in acicular or radiating fibers.This zone to the southwest contains small sill-like masses of tabularsyenite.

The major mineral is micro-to mega-perthitic feldspar surroundingthe mafic minerals which seemingly are later than the feldspars. Theprincipal mafic mineral is bluish-green arfvedsonite-riebeckite amphi-bole (Table ), sometimes mantling minor Fe-augite pyroxene. However,pyroxene is absent in some samples of this zone. Alteration of aniphi-boles to brown-red biotite is common in patches and along borders.The interesting feature of the amphibole grains is containment of a darkblue riebeckitic phase which is most common only in this unit. Someamphiboles poikilitically enclose euhedral feldspars (Figure ).

Accessories include zircon which is commonly zoned, quartz (up to12%), fluorite, calcite, FeTi-oxides, apatite and allanite.

STOP #11

TITLE : - Amphibole and Pyroxene Syenites in the Intermediate Zone

LOCATION: N W , SW , Sec. 14, T29N, R6E; Marathon 15' quadrangle: See Fig. 6.

AUTHOR: Paul E. Myers, University of Wisconsin - Eau Claire

DATE : - April , 1984

SUMMARY OF FEATURES :

Massive and flow-lineated, gray t o pinkish orange amphibole-pyroxene syenite of the Intermediate Zone i s well exposed on both sides of the road a t t h i s 1 ocal i ty. This rock closely resembles the pyroxene-bearing syenite described on page 42. The syenites just north and west of here contain l i t t l e or no pyroxene, and display complex flow structures resembl ing those a t the Old Technical Ins t i tu te (Stop #5). The amphibole syenite north of here contains abundant riebeckite. Although the dominant mafice mineral in the pyroxene- bearing syenite here i s amphibole, the pyroxene occurs as grains rimmed by amphibole. Quartz-bearing apl i tes are common in amphibole syenites they were n o t observed in the pyroxene-bearirtg syenites.

DESCRIPTION :

Whereas the amphibole i s character is t ical ly pink in outcrop, the pyroxene syeni t e i s a moderate-to-1 i ght ol ive gray w i t h islands of coarse mafics enclosed i n coarse tab1 e t s of randomly oriented fel d- spar. The amphi bole syeni t e shows considerably greater textural var- ia t ion , even a t mesoscopic scale. A1 though vein-1 i ke and i r regular masses of zoned pegmatite and a p l i t e are common in a l l outcrops, the dominant rock type i s medium-grained amphibole syenite with a f a in t t o conspicuous lamination, with or without lineation created by alignment of feldspar tablets and lensoidal c lo ts of mafic minerals--main- 1y amphibole and subordinate pyroxene. Pegmatitic phases of the amphibole syenite contain up t o 12% quartz as coarse segregations cornmon- 1y rimmed by blue (r iebecki t ic) amphibole.

In thin section, mafics are clustered in acicular or radiating f ibers . This zone t o the southwest contains small s i l l - l i k e masses of tabular syeni t e .

The major mineral i s micro-to mega-perthi t i c feldspar surrounding the mafic minerals which seemingly are l a t e r than the feldspars. The principal mafic mineral i s bluish-green arfvedsonite-riebeckite amphibole (Table ), sometimes mantl inq minor Fe-augite pyroxene. However, pyroxene i s absent in some samples of t h i s zone. A1 terat ion of amphiboles t o brown-red b io t i t e i s common in patches and along borders. The interest ing feature of the amphibole grains i s containment of a dark blue r iebecki t ic phase which i s most common only i n t h i s uni t . Some amphi boles poi kil i t i ca l ly enclose euhedral feldspars (Figure ) .

Accessories include zircon which i s commonly zoned, quartz (up to 12%), f luo r i t e , ca l c i t e , FeTi-oxides, apa t i te and a1 lan i te .

-44-

Figure 33—— Photomicrograph of amphibole syenite showingpoikilitic amphibole enclosing euhedral feldspar grains.(From Sood, Myers, and Berling, 1980, p. 34).

Figure 34-— Photomicrograph of pyroxene syenite withzoned aegirine-augite mantled by arfvedsonite. Crossedpolars. (from Sood, Myers, and Berline, 1980, p. 35).

Figure 33-- Photomicrograph of amphibole syeni te showing poi kil i t i c amphibole enclosing euhedral feldspar grains. ~ r o m Sood, Myers, and Berl ing, 1980, p. 34).

Figure 34-- Photomicrograph of pyroxene syeni t e w i t h zoned aeqi rine-auqi t e mantled by arfvedsoni t e e Crossed polars. (from Sood. Myers, and Berl ine , 1980, p. 351,

TITLE:

-45-

STOP #12

The Core Zone of the Stettin Syenite Plutori

LOCATION:

AUTHOR::

SW 1/4, SE 1/4 Sec. 2, 129N, R6E; Hanthurg151 quadra,nleWilliam Powell property. Permission required for entry.

DATE: February, 1980, April, 1984


The core of the Stettin syenite pluton comprises two distinct parts:(1) a cylindrical core margin of indistinctly banded or lineated, medium-grained nepheline syenite and (2) an inner core of pyroxene syenite. Bentand crushed feldspar grains and a crude southeast-dipping layering wereformed during or after emplacement of the core margin. The nepheline syenitecore margin produced a pronounced donut-shaped magnetic anomaly about one milein diameter. Drilling by Bear Creek Mining Company in the southeast corner ofthe inner core retreived about 250 feet of core classified by company geologistsas larvikite. No carbonatite has been found, although the agpaitic trend ofthe rocks here suggests that such a carbonatite is possible (Koellner, 1974,p. 144).

DESCRIPTION:

The nepheline syenite of the core margin here is indistinctly banded orlineated. The weathered surface is pale yellowish gray with pitting due todifferential weathering of the nepheline. The fresh nepheline is pale greenishbrown and occurs as well-oriented, subhedral to euhedral grains enclosed bytablets of feldspar up to 2 cm long. The feldspars, nepheline, and islandsof mafic minerals are elongated in a plane dipping southeast at between 60 and70g. This lamination is not parallel to the outer edge of the core margin atthis location. Bent and broken feldspar and nepheline grains and lenticulationof mafic mineral clusters suggest shearing during or after intrusion.

Paul E. Myers , University of Wisconsin — Eau Claire

STOP #12

TITLE: The Core Zone of the S te t t in Syenite Pluton

LOCATION: SW 1/4, SE 1/4 Sec. 2, T29N, R6E; Hmburg 15' quadrangle William Powell property. Permission required for entry.

AUTHOR,: Paul E. Myers, University o f Wisconsin - Eau Claire

DATE : February, 1980, Apri 1 , 1 984


The core of the S te t t in syenite pluton comprises two d i s t inc t parts: (1 ) a cylindrical core margin of indis t inct ly banded or l ineated, medium- grained nepheline syenite and (2) an inner core of pyroxene syenite. Bent and crushed feldspar grains and a crude southeast-dipping layering were formed during or a f t e r emplacement of the core margin. The nepheline syenite core margin produced a pronounced donut-shaped magnetic anomaly about one mile i n diameter. Dril l ing by Bear Creek Mining Company in the southeast corner of the inner core retreived about 250 f ee t of core classif ied by company geologists as 1 arvi ki te . No carbonati t e has been found, a1 though the a pal t i c trend of the rocks here suggests tha t such a carbonatite i s possible Koellner, 1974, p. 144).

?

DESCRIPTION:

The nepheline syenite of the core margin here i s indis t inct ly banded or lineated. The weathered surface i s pale yellowish gray with p i t t ing due to different ial weathering of the nepheline. The fresh nepheline i s pale greenish brown and occurs as we1 1-oriented, subhedral to euhedral grains enclosed by tablets of feldspar up to 2 cm long. The feldspars, nepheline, and islands ofmaf ic minerals are elongated in a plane dipping southeast a t between 60 and 70 . This 1 amination i s not parall el to the outer edge of the core margin a t t h i s location. Bent and broken feldspar and nepheline grains and lenticulation of mafic mineral c lusters suggest shearing during or a f t e r intrusion.

-46-

The dominant mineral is tabular microperthite (60% orthoclase with 40%oligalase ribbons). An additional 25% of the rock is subhedral to euhedralnepheline, which is partially altered to cancrinite. Mg-rich pyroxene andpleochroic, olive brown amphibole are of about equal abundance and make upabout 20-30% of the rock. Accessory (2-5%) Mg-rich olivine and dark.brownbiotite accompany the other mafic minerals in lenticular clusters and islandsoccurring interstitially in the nepheline syenite. The biotite partially rimsthe amphibole and was probably formed at a late stage of crystallization.

This unit produced a pronounced, donut-shaped magnetic anomaly about onemile in diameter. Wiedman (1907, p. 251) reports unusually large and abundantmagnetite octahedra from streams northwest of here. The magnetite is apparentlyassociated most closely with the olivine.

The dominant mineral i s tabular microperthite (60% orthoclase w i t h 40% oligoclase ribbons). An addit ional 25% of the rock i s subhedral t o euhedral nepheline, which i s p a r t i a l l y a l t e red t o cancr ini te . Mg-rich pyroxene and pleochroic, o l ive brown amphibole a r e of about equal abundance and make up about 20-30% of the rock. Accessory (2-5%) Mg-rich o l iv ine and dark. brown b i o t i t e accompany the other mafic minerals i n l en t i cu l a r c lu s t e r s and is lands occurring i n t e r s t i t i a l l y i n the nephel ine syeni t e . The b i o t i t e pa r t i a l l y rims the amphibole and was probably formed a t a l a t e s tage of c ry s t a l l i z a t i on .

This u n i t produced a pronounced, donut-shaped magnetic anomaly about one mile i n diameter. Wiedman (1907, p. 251 ) reports unusually large and abundant magnetite octahedra from streams northwest of here. The magnetite i s apparently associated most c losely w i t h the o l iv ine .

-47-.

PETR0CHEMISTRY *

Chemical compositions of the Stettin rocks are presented in Table 7

Table 8compares average compositions of the Stettin rocks to those of Nockold's(1954). The average of the Stettin nepheline syenites show distinct differencesfrom Nockold's average syenite. These Stettin samples, while only slightlyhigher in silica, are lower in Al903 and NA2O and higher in FeO, CaO and P205.The amphibole and pyroxene syeniths, also slightly higher in silica thanNockold's average syenite, are lower in Al20., MgO, CaO and 1(90, while higherin FeO, NA2O and MnO. The differentiation i?dices (DI normtive quartz +orthoclase + albite + nepheline + leucite + kalsilite) (Thornton and Tuttle,1960) for these Stettin rocks are given in Table 9 . The average DI for theserocks is 84.7, which represents a high degree of differentiation. However,nepheline syenites have the highest DI of 88.9 and 93.9 respectively, indicatingthe greatest extent of differentiation among these rocks.

The agpaitic indices of the Stettin samples are shown in Figure 35—A.Rocks of lower Si09 content, the nepheline bearing rocks, have lower agpaiticindices than the mre silica rich rocks. This is a reflection of the higheralumina content, due to the presence of nepheline, in the nepheline syenites.The ratio Na20/K20 versus Si02 (Figure 35C) increases with increasing Si02.This diagram shows two trends suggesting that the Stettin rocks belong totwo series. Amphibole and pyroxene syenites appear to follow a continuousdifferentiation sequence. (Figures 35E-G). C.I.P.W. normative compositionsare presented in Table 9 The normative compositions of the analyzed Stettinrocks were calculated in terms of NaA1SiO4, KA1SiO4 and SiO and are plottedin the systems NaA1Si0 - KalSiO4 - Si02 at 1000 bars H

0Figure 36). All of

the rocks fall within the low temperature trough. 2

TABLE 7

ROCK TYPE Anphibole Syenite —nESyenite—

jabularSyenite Nepheline Syenite—

Sarrale 9—

Sb2

A1203

Fe203

FeO

MgO

CaO

K0

620

CO2

1102

'05nO

S

(2r02

Cl

BaO

Rb(pprn)

Sr(ppm)-

—10—

66.10

13.24

2.61

4.12

0.43

0.70

5.92

4.31

0.73

0.38

70

65.20

15.59

2.36

2.22

0.01

0.50

6.92

5.11

0.83

0.35

503—64.70

15.86

2.45

2.10

0.02

0.95

7.07

5.19

0.10

0.36

108—61.95

16.04

3.13

2.70

0.08

1.10

6.51

5.51

1.95

0.40

6+504—59.75

• 16.23

2.55

5.66

0.14

2.15

5.97

5.67

0.51

0.22

65

61.S0**

16.62

5.20

1.68

0.24

1.43

6.49

5.15

0.63

0.17

46—5745

16.93

2.58

5.98

0.21

2.64

6.71

5.02

0.98

0.18

2

5695

21.02

2.93

2.12

0.07

0.51

7.81

5.99

1.43

0.40

92—54.10

16.32

3.41

7.08

1.22

4.03

5.01

4.84

0.77

0.09

0.72

0.11

0.23

0.010

0.102

0.03

—

0.42

0.04

0.12

0.004

0.165

0.215

0.013

0.071

199.

44

0.27

0.06

0.15

0.003

0.260

0.024

—

0.32

0.07

0.18

0.008

0.171

0.143

0.010

0.150

152.

105.—

0.75

0.13

0.26

0.034

0.11

0.010

0.160

66.

174.—

0.31

0.07

0.22

0.009

0.100

0.345

0.103

133.

109.

0.59

0.13

0.30

0.023

0.140

0.241

0.02

0.086

115.

57.—

0.38

0.50

0.07

0.000

0.001

0.025

1.32

0.49

0.29

0.044

0.079

0.105

0.02

0.208

102.

345.—.* Analyst-K. Racial. University of Manitoba**Tabular

yeniteSooci, vers, and Berlin, 1980.

INTERMEDIATE ZONE

CHEMICAL COMPOSITION OF STETTIN PLUTON ROCKSCORE WALL ZONEZONE

*Modjfjed fror.i

PETROCHEMISTRY*

Chemical composit ions o f t he S t e t t i n rocks are presented i n Table 7 Table 8 compares average composit ions o f t he S t e t t i n rocks t o those o f Nockold 's (1954). The average o f t h e S t e t t i n nephel i n e syeni t e s show d i s t i n c t d i f f e rences from Nockold 's average syen i te . These S t e t t i n samplesy w h i l e o n l y s l i g h t l y h igher i n s i l i c a * a re lower i n A1 O3 and NA 0 and h ighe r i n FeO* CaO and P205. The amphibole and pyroxene syen i t$sy a l s o s ? i g h t l Y h ighe r i n s i l i c a than Nockold's average s y e n i t e y a re lower i n A1 0 MgOy CaO and K2OY w h i l e h igher i n FeO* NA20 and MnO. The d i f f e r e n t i a t i o n 2 i d d i c e s (DI = normat ive qua r t z + or thoc lase + a1 b i t e + nephel ine + l e u c i t e + k a l s i l i t e ) (Thornton and T u t t l e * 1960) f o r these S t e t t i n rocks are g iven i n Table 9 . The average D I f o r these rocks i s 84. 7* which represents a h igh degree o f d i f f e r e n t i a t i o n . However* nephel ine syen i tes have the h ighes t D I of 88.9 and 93.9 r e s p e c t i v e l y y i n d i c a t i n g the g rea tes t e x t e n t o f d i f f e r e n t i a t i o n among these rocks.

The a g p a i t i c i n d i c e s o f the S t e t t i n samples a re shown i n F igure 35-A. Rocks o f lower Si02 con ten t * t he nephel ine bear ing rocks* have lower a g p a i t i c i nd i ces than t h e more s i l i c a r i c h rocks. This i s a r e f l e c t i o n o f t h e h igher alumina con ten t * due t o t h e presence o f nephe l ine* i n t he nephel ine syen i tes . The r a t i o Na20/K20 versus Si02 (F igure 35C) increases w i t h i n c r e a s i n g Si02. This diagram shows two t rends suggest ing t h a t t h e S t e t t i n rocks belong t o two se r i es . Amphibole and pyroxene syen i tes appear t o f o l l o w a cont inuous d i f f e r e n t i a t i o n sequence. (F igures 35E-G). C . I .Paw. normative composit ions are presented i n Table 9 The normative composit ions o f t h e analyzed S t e t t i n rocks were c a l c u l a t e d i n terms o f NaA1SiO4Â KAlSi04 and S i O and are p l o t t e d i n t h e s y s t e m s N a A l S i 0 - K a l S i 0 4 - S i 0 2 a t 1 0 0 0 b a r s P f ~ i c j u r e 36). A11 o f t he rocks f a l l w i t h i n ?he low temperature trough.

TABLE 7

- ROCK TYP - Q w l e #

s i o2

%O3

Fe2Â°

FeO

M90

cao

h2Â

'5O

"zO c02

T i o2

'2'5

Nlo

s

lm2

Cl

Bao

P . ~ ( P P ~

{EMICAL COMPOSITIO INTERMEDIATE ZONE

Anphibole Syeni t e - 503 -

64.70

15.86

2.45

2.10

0.02

0.95

7.07

5.19

0.70

0.36

0.27

0.06

0.15

0.003

0.260

0.024

- o f Mar

OF - CORE ZONE -

froxene yeni te - 6+504 - 59.75

16.23

2.55

5.66

0.14

2.15

5.97

5.67

0.51

0.22

0.75

0.13

0.26

0.034

0.11

0.01c

0.16(

66.

174. - ~na1ys.t-K. Ramla1 , Univers i ty

**la u la r yeni te *Modi f ied f m r - i S O O C ! ~ F!"ersy and Uerl i r ~ * 1989.

l i n e Syenite

2 92 I

56.95 54.10

21.02 16.32

2.93 3.41

2.12 7.08

0.07 1.22

0.51 4.03

7.81 5.01

5.99 4.84

1.43 0.77

0.40 0.09

0.38 1.32

0.50 0.49

0.07 0.29

0.000 0.044

0.001 0.079

0.105

0.025 0.02

0.208

102.

345.

-48-

TABLE 8

COMPARISON OF CHEMICAL COMPOSITIONS OF STETTIN WITH NOCKOLDS (1954) AVERAGES

Average StettinNepheline Syenlte

Average NephelineSyenite (Nockolds,1954)

Average StettinSyenite

Average Syenite(Nockolds, 1954)

S102

Al203

Fe203

FeO

56.17

18.09

2.97

5.06

55.38

21.30

2.42

2.00

63.54

15.39

2.62

3.36

61.86

16.91

2.32

2.63

MgO 0.50 0.57 0.14 0.96

CaO 2.39 1.98 1.08 2.54

Na20

1(20

1120

1102

P205

InÔ

6.78

5.28

1.06

0.76

0.37

0.22

8.84

5.34

0.96

0.66

0.19

0.19

6.49

5.16

0.94

0.50

0.08

0.19

5.46

5.91

0.53*

0.58

0.19

0.11

* Includes only H20from Socd, Myers, and Berlin, 1980, p. 48.

TABLE 9

C.I.P.W. NORMATIVE COMPOSITIONS OF THE STETTIN ROCKS

INTERMEDIATE ZONE CORE ZONE WALL ZONE

ROCK TYPE Amphibole Syenlte PyroxeneSyenite

TabularSyenite

Nepheline Syenite

Sample Numbers* 10 77 503 100 6 and 504 65 46 2 92

Q 12.44% 4.86% 2.80% 1.79% 1.80%Or 25.61 30.06 30.62 32.28 33.40% 30.62 29.50% 35.62% 28.39%Ab 44.05 51.92 52.97 51.92 47.72 52.44 56.48 38.39 34.16An 0.88 2.22 1.47 2.11 4.26Ne 1.28 7.93 14.86 8.1001 3.41 3.00 0.96 4.67My 7.22 2.36 1.36 1.65Ac 5.31 5.91 5.89 2.70Dl 1.89 3.88 2.70 6.66 9.82 10.65Mt 1.04 0.51 0.54 3.28 3.70 5.06 3.70 4.17 4.86Ii 1.36 0.76 0.46 0.61 1.36 0.61 1.06 0.76 2.43Pr 0.12 0.01 0.01 0.01 0.06 0.02 0.06 0.06Ru 0.02Nm 1.62C 0.91Ap 0.34 0.10 0.13 0.17 0.34 0.17 0.34 0.13 1.01Z 0.18 0.18 0.37 0.18 0.02 0.15 0.18 0.001 0.11Hl 0.06 0.02 0.04 0.02 0.02 0.58 0.03 0.06 0.04Tn 1.70CC 0.90 0.50 0.20DI 82.1 86.8 86.4 86.0 82.4 84.9 93.9 88.9 70.6

*Tabular Syenite

-48-

TABLE 8 COMPARISON OF CHEMICAL COMPOSITIONS OF STETTIN WITH NOCKOLDS (1954) AVERAGES

. sio2

A1203

Fe203

FeO

w CaO

Na20

%O A

"2O

Ti02

p2Â°

M ~ O '

Average Nephel b e Syenite (~ockolds.1954)

Average S t e t t i n Syenl t e

Average Syeni t e (Nockolds, 1954)

Average S t e t t i n Nephel i ne Syeni t e

$ncludes on ly H20 from Soc,d, Myers, and Berlin, 1980, p. 48.

TABLE 9 C.I.P.W. NORMATIVE COMPOSITIONS OF THE STETTIN ROCKS

INTERMEDIATE ZONE CORE ZONE WALL ZONE

1 ROCK TYPE Amphi bole Syeni t e Pyroxene Sveni t e

Tabular Syeni t e -

65

Nephel i n e Syeni t e

Sample Numbers* L 6 and 504

*Tabular Syeni t e

-49-

I'

108

Q2o -

7)0 ..44 6 1 0 1080 46 65'a

02

0 K09

92

--08

_________________ ________________

30 60 70 50 60 70

S.O 94 SiO 94

IS®4 20

* I0 3

2 503108

O 2l246 4600

0092

97 '10I 0

50 60 70 50 60 70

0.05%

0

o ®20 °

108

65

0 '892 6 77

. IS

C92

l0

__________________________

IC I

60 70 50 60 70

SiO % 507%

92i 0

3 .5

0460

1.0I2

2 S

046 \l0

__

,l00.5

20 " •"20 65 "503

C 0.C50 60 70 50 60 70

5102 94 502 96

Figure 35-- Chemical trends of major elements versus Si02for Stettin pluton rocks (from Sood and others, 1980, p0 50)

Figure 35-- Chemical t r e n d s o f major e l e m e n t s v e r s u s S i 0 2 f o r S t e t t i n p lu ton rocks (from Sood and o t h e r s , 1980, p o 50)

-50-

DISCUSSION

M. K. Sood and L. A. Berlin

Due to chemical and mineralogical heterogeneity, the origin of alkalineigneous rocks is, in many cases, very complex and may be the result of severalprocesses. Experimental studies of chemically equivalent synthetic silicatesystems (Bailey and Schairer, 1966; Hamilton and MacKenzie, 1965; Schairer,1967; Sood and Edgar, 1972; Sood, Platt and Edgar, 1970; Tuttle and Bowen,1958) have provided a physicochemical framework to explain the crystallizationbehavior of alkali magmas.

Any petrogenetic model for the formation of alkaline rocks of the Wausauarea must take into account:

1) the zoned nature of the complex

2) the presence of quartz-bearing aplitic and pegmatitic stages inthe intermediate ring of amphibole syenite;

3) the fenitized zone surrounding the pluton;

4) the presence of volatile bearing minerals (flourite, calcite,apatite) in most syenites, and in the quartz monzonite 'core" (?)of the Wausau pluton;

5) major and trace element geochemistry of the syenites.

Consideration with Respect to the System Nepheline-Kalsilite-Silica

In Figure 36 normative composition of the Stettin rocks is plotted in thesystem Nepheline-kalsilite-silica at 1Kb H along with the composition ofthe rocks from Kangerdlugssuag intrusion, 2 East Greenland (Wager, 1965).These analyses may be interpreted to show a trend of silica depletion awayfrom the Si02 apex.

Rocks of the Intermediate Zone of amphibole syenite plot in the alkalifeldspar-quartz region, near the alkali feldspar join, while pyroxene syenitesof the Core Zone plot just below the alkali feldspar join. The positions ofthese syenites in the field show a silica depletion trend toward the centerof the complex.

From Figure 36, it appears that the trend of these amphibole and pyroxenesyenites is up the alkali feldspar surface and 1overt' the thermal barrier,which is similar to the interpretation by Wager (1965) for the nordmarkites,pulaskites, and foyaites of the alkaline Kangerdlugssuaq intrusion.

(In the nepheline-kalsilite-silica system at 5 Kb H these rock websplot close to the feldspar cotectic or nephiline-feldspar 2 cotectic. Thisis in agreement with mineral paragenetic and textural relations.) Furtherinterpretations await the accumulation of additional data, especially on theWausau pluton.

DISCUSSION M. K. Sood and L. A. Berlin

Due t o chemical and mineralogical heterogeneityy the or igin of a lka l ine igneous rocks i s in many casesy very complex and may be the r e s u l t of several processes. Experimental s tudies of chemically equivalent synthet ic s i1 i c a t e systems (Bai 1 ey and Schai r e r y 1966; Hami 1 ton and MacKenziey 1965; Schairer 1967; Sood and Edgary 1972; Sood, P l a t t and Edgary 1970; Tut t l e and Boweny 1958) have provided a physicochemical framework t o explain the crysta l 1 iza t ion behavior of a1 ka1 i magmas.

Any petrogenetic model f o r the formation of a lka l ine rocks of the Wausau area must take in to account:

1 ) the zoned nature of the complex

2 ) the presence of quartz-bearing a p l i t i c and pegmatitic stages i n the intermediate r ing of amphibole syeni te ;

3) the feni t ized zone surrounding the pluton;

4 ) the presence of vo1 a t i l e bearing mineral s ( f1 ouri t e ca1 c i t e a p a t i t e ) i n most syen i tesy and in the quartz monzonite "core" ( ? ) of the Wausau pluton;

5) major and t r ace element geochemistry of the syenites.

Consideration w i t h Respect t o the System Nephel ine-Ka1 s i 1 i te-Si 1 ica

In Figure 36 normative composition of the S t e t t i n rocks i s p lot ted i n the system Nepheline-kalsilite-silica a t 1Kb PH along w i t h t he composition of the rocks from Kangerdl ugssuag intrusion 2 East Greenland (Wagery 1965). These analyses may be in terpreted t o show a trend of s i l i c a depletion away from the Si02 apex.

Rocks of the Intermediate Zone of amphibole syeni te p lot in the a lka l i fe1 dspar-quartz region, near the a1 ka1 i fe1 dspar j o iny w h i 1 e pyroxene syeni t e s of the Core Zone plot j u s t below the a lka l i feldspar join. The posit ions of these syeni tes i n the f i e l d show a s i1 ica depletion trend toward the center of the complex.

From Figure 36, i t appears t h a t the trend of these amphibole and pyroxene syenites i s up the a1 kali feldspar surface and "over" the thermal b a r r i e r y which i s s imi la r t o the in te rpre ta t ion by Wager (1965) fo r the nordmarkitesy pulaski t e s y and foyai t e s of the a1 kal ine Kangerdl ugssuaq int rus ion.

(In the nephel ine-kalsi l i t e - s i l i c a system a t 5 Kb PH o y these rock webs plot c lose t o the fe ldspar co tec t ic o r nephiline-feldspar 2 co tec t ic . This i s in agreement w i t h mineral paragenetic and textural r e l a t i ons . ) Further in te rpre ta t ions await the accumulation of addit ional da tay especia l ly on the Wausau p1 uton.

Figure 36-- Normative compositions of the Stettin rocks (closed circles)and the alkaline rocks of the Kangerdlugssuaq intrusion, East Greenland(open circles) (Wager, 1965) plotted in the system NaA1SiO4 - KA1SiO4 -Si02 at H2O = 1000 bars (Fudali, 1963; Hamilton and MacKenzie, 1965).

—51—

S i0100

5

K A IS 1308

Nlephehne ss

100

N aA IS 104

20 30 40

Weight per cent.

10

50 60 70 80 900

100

KAISIO4

N e p h e l i n e ss / $6 / 0 1 /' 8 Kals i l i te ss 0 / 2

/ / ,, ,, . *-

/ - ' / J V - 0 10 20 30 4 0 5 0 6 0 7 0 80 9 0 100

d o

s i o 4 W e ~ g h t per cent. KAlSiO4

Figure 3 6 . - - Normative compositions of t h e S t e t t i n rocks ( c lo sed c i r c l e s ) and t h e a l k a l i n e rocks o f t h e Kangerdlugssuaq i n t r u s i o n y Eas t Greenland (open c i r c l e s ) (Wager, 1965) p l o t t e d i n t h e system NaA1Si04 - KAlSiO4 - Si02 a t P H ~ ~ = 1000 b a r s (Fudal i 1963; Hamil ton and MacKenzie 1965).

-52—

How could such inward silica depletion be caused? Two possible explanationsare:

(1) Loss of the volatile phase ineuilibrium with the melt. Such avolatile phase has alumina, alkali, and silica in the same pro-portion as feldspars (Tuttle & Bowen, 1958; Mackenzie, 1960).The presence of aplitic and pegmatitic phases and fenitizationof the surrounding volcanics may be a reflection of separationof volatiles into a gaseous phase and eventual loss. The plotof the Stettin rocks close to cotectics in pertinent syntheticsystems may be indicative of crystallization of major phaseswithin narrow temperature limits. Short crystallization intervalsare also related to silica and alkali content which controlvolatile distribution in liquid and gaseous phases (Sood & Edgar,1970; Kogarko & Rhyaschikov, 1961).

(2) The substitution of Fe3 A13 in feldspars may contribute tosilic depletion with crystallization of iron-rich albite(NaFe 3Si20). Only a small amount of Fe-Al substitution isnecessary th fix silica and cause the liquid to shift fromsilica saturated to silica undersaturated trend (Bailey & Schairer,1966). The general iron-rich and alumina-deficient nature ofthe syenites in comparison to Nockold's (1954) averages and alimited Fe-content of feldspars favor such substitution.

The Nepheline syenite in the Stettin pluton may, therefore, representlast residual liquids injected into the sheared wall zone.

It may be concluded that alkaline rocks of Marathon County representa "genetically related comagmatic series." The study of silicate systemsand melting relations of rocks have amply demonstrated that magma composi-tion lies close to the univariant lines or the invariant points, and veryslight changes in initial liquid composition can give decidedly distinctliquid trends. Compositional differences in these alkaline rocks may berelated to slight changes in magma composition by fractional crystallizationor by wallrock assimilation, or both. It is important to further refinetheir genetic and tectonic relations. Systematic geochemical data both onrocks and minerals are needed to assess if these rocks are formed frommantle derived magmas (tentatively note the low Rb and Sr contents forWausau rocks) which reached crust through recurrent fracture systems. Suchinformation will also be Useful in the estimation of economic mineral potentialof this area. Such rocks form in environments favorable to the concentrationof a wide variety of elements.

How could such inward s i l i c a depletion be caused? Two possible explanations are :

(1 ) Loss of the v o l a t i l e phase i n equilibrium with the melt. Such a v o l a t i l e phase has alumina, a1 kali and s i l i c a in the same pro- portion a s fe ldspars (Tu t t l e & Bowen? 1958; Mackenzie? 1960). The presence of ap1 i t i c and pegmati t i c phases and feni t i z a t i on of the surrounding volcanics may be a re f l ec t ion of separation of vo l a t i l e s i n to a gaseous phase and eventual loss . The p lo t of the S t e t t i n rocks c lose t o co tec t i cs i n per t inent synthet ic systems may be indicat ive of c ry s t a l l i z a t i on of major phases within narrow temperature 1 imi ts. Short crys ta l 1 iza t ion i n t e rva l s a r e a l so re la ted t o s i l i c a and a lka l i content which control v o l a t i l e d i s t r ibu t ion i n l i qu id and gaseous phases (Sood & Edgar? 1970; Kogarko & Rhyaschi kov 1961 ) . The subs t i tu t ion of ~ e ' ~ A I ' ~ i n fe ldspars may contr ibute t o s i1 icq depletion w i t h c rys ta l1 iza t ion of iron-rich a1 b i t e (NaFe 3 ~ i 0 ) . Only a small amount of Fe-A1 subs t i tu t ion i s necessary2t8 f i x s i l i c a and cause the l i qu id t o s h i f t from s i l i c a sa tura ted t o s i l i c a undersaturated trend (Bailey & Schairer? 1966). The general i ron-rich and alumina-deficient nature o f the syeni tes in comparison t o Nockold's (1954) averages and a 1 imited Fe-content of fe1 dspars favor such subs t i tu t ion .

The Nepheline syeni te in the S t e t t i n pluton may, the re fore? represent l a s t residual l iqu ids in jected in to the sheared wall zone.

I t may be concluded t h a t a1 kal ine rocks of Marathon County represent a "genet ica l ly re la ted comagmatic se r ies . ' ' The study of s i l i c a t e systems and me1 t i ng re1 a t ions of rocks have amply demonstrated t h a t magma composi - t ion l i e s c lose t o the univariant l i n e s o r the invar ian t points? and very s l i g h t changes in i n i t i a l l i qu id composition can give decidedly d i s t i n c t 1 i q u i d t rends . Composi t i ona1 di fferences in these a1 ka1 ine rocks may be re la ted t o s l i g h t changes in magma composition by f ract ional c ry s t a l l i z a t i on o r by wallrock ass imila t iony o r both. I t i s important t o f u r t he r re f ine t h e i r genetic and tec tonic re la t ions . Systematic geochemical data both on rocks and minerals a r e needed t o assess i f these rocks a r e formed from mantle derived magmas ( t en t a t i ve ly note the low Rb and Sr contents f o r Wausau rocks) which reached c ru s t through recurrent f r a c tu r e systems. Such information wi l l a l so be useful in the estimation of economic mineral potential . of this area. Such rocks form i n environments favorable t o the concentration of a wide var ie ty of elements.

—53—

DISCUSSION

P.E. Myers

The four plutons of the Wausau syenite complex share many characteristics oflithology and chemistry which link them in age and origin. Moreover, they showa general differentiation trend from the peralkaline Stetting syenite with itsnepheline syenite rim and cor margin through the Wausau and Rib Mountain plutonswhich are characterized by a southeastward increase in silica, and finally tothe Ninemile pluton with its core rim of aplitic granite. Although there is noobvious structural control of the complex, it does occupy a major flexure nearthe boundary between mafic and intermediate Early Proterozoic metavolcanic rockson the west and felsic to intermediate metavolcanic rocks on the east. Thecomplex lies only about 25 miles west of the exposed western edge of the coevalWolf River batholith, and the Ninemile granite shares many mineralogical andchemical characteristics with the Wolf River batholith: they both have rapakiviaffinities. Aspects of concentric zoning of each pluton permit some conclusionsas to intrusion sequence, although problems remain as to the actual mechanism(s)of emplacement. Xenoliths are nearly absent in the Stettin pluton, the oldestin the sequence, possibly because the conduit was developed more rapidly and/orbecause the Stettin pluton has been more deeply eroded. The occurrence of meta-volcanic xenoliths closely resembling rocks exposed nearby along with xenolithsof high—grade mica schists and quartzites suggests considerable vertical mixingof wallrock fragments along the walls of the Wausau and Rib Mountain plutons.These plutons were penetrating a high-grade metamorphic terrane at depth, afactor suggesting the presence of an earlier Proterozoic succession beneath thevolcanic rocks. The problem of the "two Proterozoic successions" has beenrecently discussed by LaBerge and Myers (1984.) The xenoliths in the intermediatezones of the Wausau and Rib Mountain plutons indicates the initial emplacementof syenite magma during a first high—volume surge, probably with venting at thesurface. The first-surge magma then crystallized inward from the dylindricalwalls, forming the wall zones. Then (for the Wausau and Rib Mountain plutons),with repeated resurgence and collapse along cylindrical shear zones, the deeperwallrock fragments were mixed with those carried downward by an earlier collapsephase. This would explain the tendency for concentric elongation of xenolithsand their zonal distribution. If the southwesterly track of the plutonic sequencecan be generalized, it is possible that other syenitic plutons exist in Wisconsin,but are obscured beneath younger rocks and glacial deposits. The aeromagnetic mapof Wisconsin shows many circular anomalies similar to those characterizing theWausau syenite (See LaBerge and Myers, 1983, Plates 1 and 2).

Features indicating shallow intrusion depth are: (1) abundance of nearlyhorizontal pegmatite pods and miarolitic cavities in the Ninemile granite;(2) concentric, cylindrical, discordant structure of the plutons, and (3) lowmetamorphic grade and relative simplicity of structure in wallrocks.

Some problems exist concerning the age and genetic relationships of thegranite aplite, which occupies a large area outside the plutons, but is confinedmainly to the region adjacent to the Ninemile pluton. In essence, the apliteis a finer grained variety of the Ninemile granite. However, miarolitic cavitiesand pegmatite pods and dikes are very rare in the aplite. However, the aplitemay represent an early phase of granite intrusion which preceded emplacement ofthe Nineniile granite. The aplite contains plagioclase phenocrysts of euhedralplagioclase and tends to be leucocratic: biotite is usually the only mafic mineral,and its abundance is usually less than 5 percent. Mantling of K-feldspar by sodicplagioclase is common. It is tentatively concluded that the granite aplite is anearly intrusive phase of the Ninemile granite.

DISCUSSION

P.E. Myers

The four p l u tons o f t h e Wausau s y e n i t e complex share many c h a r a c t e r i s t i c s o f l i t h o l o g y and chem is t r y which l i n k them i n age and o r i g i n . Moreovery t h e y show a genera1 d i f f e r e n t i a t i o n t r e n d f rom t h e pe ra l k a l i n e S t e t t i n g s y e n i t e w i t h i t s nephe l ine s y e n i t e r i m and c o r marg in th rough t h e Wausau and R ib Mountain p l u t o n s which a r e c h a r a c t e r i z e d by a southeastward i nc rease i n s i l i c a * and f i n a l l y t o t h e N inemi le p l u t o n w i t h i t s co re r i m o f a p l i t i c g r a n i t e . A l though t h e r e i s no obv ious s t r u c t u r a l c o n t r o l o f t h e complexy i t does occupy a ma jo r f l e x u r e near t h e boundary between m a f i c and i n t e r m e d i a t e Ear l ' y P r o t e r o z o i c metavo lcan ic rocks on t h e west and f e l s i c t o i n t e r m e d i a t e metavo lcan ic rocks on t h e eas t . The complex l i e s o n l y about 25 m i l e s west o f t h e exposed western edge o f t h e coeval Wolf R i v e r b a t h o l i t h * and t h e N inemi le g r a n i t e shares many m i n e r a l o g i c a l and chemical c h a r a c t e r i s t i c s w i t h t h e Wol f R i v e r b a t h o l i t h : t h e y bo th have r a p a k i v i a f f i n i t i e s . Aspects o f c o n c e n t r i c zon ing o f each p1 u ton pe rm i t some conc lus ions as t o i n t r u s i o n sequence* a l though problems remain as t o t h e ac tua l mechanism(s) o f emplacement. Xeno l i t hs a re n e a r l y absent i n t h e S t e t t i n p l u t o n y t h e o l d e s t i n t h e sequence* p o s s i b l y because t h e c o n d u i t was developed more r a p i d l y and/or because t h e S t e t t i n p l u t o n has been more deeply eroded. The occurrence o f meta- v o l c a n i c xenol i t h s c l o s e l y resembl i n g rocks exposed nearby a l ong w i t h xenol i t h s o f h igh-grade mica s c h i s t s and q u a r t z i t e s suggests cons ide rab le v e r t i c a l m i x i n g o f w a l l r o c k fragments a l ong t h e w a l l s o f t h e Nausau and R ib Mountain p l u tons . These p l u tons were p e n e t r a t i n g a h igh-grade metamorphic t e r r a n e a t depth, a f a c t o r sugges t ing t h e presence o f an e a r l i e r P r o t e r o z o i c success ion beneath t h e v o l c a n i c rocks. The problem o f t h e " two P r o t e r o z o i c s u ~ c e s s i o n s ' ~ has been r e c e n t l y d iscussed by LaBerge and Myers (1984.) The x e n o l i t h s i n t h e i n t e r m e d i a t e zones o f t h e Wausau and R ib Mountain p l u tons i n d i c a t e s t h e i n i t i a l emplacement o f s y e n i t e magma d u r i n g a f i r s t high-volume surge* p robab l y w i t h v e n t i n g a t t h e su r face . The f i r s t - s u r g e magma then c r y s t a l 1 i z e d inward f rom t h e c y l i n d r i c a l w a l l s , fo rm ing t h e w a l l zones. Then ( f o r t h e Wausau and R ib Mountain p1 u tons) w i t h repeated resurgence and c o l l a p s e a l o n g c y l i n d r i c a l shear zonesy t h e deeper w a l l r ock fragments were mixed w i t h those c a r r i e d downward by an e a r l i e r c o l l apse phase. Th i s wou1 d e x p l a i n t h e tendency f o r c o n c e n t r i c e l o n g a t i o n o f xenol i t h s and t h e i r zonal d i s t r i b u t i o n . I f t h e sou thwes te r l y t r a c k o f t h e p l u t o n i c sequence can be gene ra l i zed * i t i s p o s s i b l e t h a t o t h e r s y e n i t i c p l u t o n s e x i s t i n Wiscons iny b u t a r e obscured beneath younger rocks and g l a c i a l depos i t s . The aeromagnet ic map o f Wisconsin shows many c i r c u l a r anomalies s i m i l a r t o those c h a r a c t e r i z i n g t h e Wausau s y e n i t e (See LaBerge and Myers 1 983* P l a t e s 1 and 2) .

Features i n d i c a t i n g sha l l ow i n t r u s i o n depth a re : ( 1 ) abundance o f n e a r l y h o r i z o n t a l pegmat i te pods and m i a r o l i t i c c a v i t i e s i n t h e N inemi le g r a n i t e ; ( 2 ) c o n c e n t r i c * c y l i n d r i c a l d i s co rdan t s t r u c t u r e o f t h e p l u t o n s y and ( 3 ) low metamorphic grade and r e l a t i v e s i m p l i c i t y o f s t r u c t u r e i n wa l l r ocks .

Some problems e x i s t concern ing t h e age and gene t i c r e l a t i o n s h i p s o f t h e g r a n i t e a p l i t e * which occupies a l a r g e area o u t s i d e t h e p l u t o n s y b u t i s con f i ned m a i n l y t o t h e r e g i o n ad jacen t t o t h e N inemi le p l u ton . I n essence* t h e a p l i t e i s a f i n e r g ra i ned v a r i e t y o f t h e N inemi le g r a n i t e . Howevery m i a r o l i t i c c a v i t i e s and pegmat i te pods and d i kes a r e ve r y r a r e i n t h e a p l i t e . However* t h e a p l i t e may rep resen t an e a r l y phase o f g r a n i t e i n t r u s i o n which preceded emplacement o f t h e Ninemi l e g r a n i t e . The ap1 i t e con ta i ns p l a g i o c l ase phenocrysts o f euhedral p l a g i o c l a s e and tends t o be l e u c o c r a t i c : b i o t i t e i s u s u a l l y t h e o n l y m a f i c m ine ra l , and i t s abundance i s u s u a l l y l e s s than 5 percent . M a n t l i n g o f K - f e l dspa r by sod i c p l a g i o c l a s e i s common. I t i s t e n t a t i v e l y concluded t h a t t h e g r a n i t e a p l i t e i s an e a r l y i n t r u s i v e phase o f t h e N inemi le g r a n i t e .

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Another problem concerns two quartz monzonite porphyry plugs (See Figure 5).These small plug—like bodies consist of a light—colored, aphanitic matrix andlarge K—feldspar phenocrysts showing partial fragmentation and resorption.Quartz phenocrysts are granular and anhedral. These plutons are strung outalong a NW-SE axis across Marathon County and were intruded through post—syenitefaults. They are interpreted as being the "last gasp" of the Ninemile graniteemplacement episode.

Whole rock and trace element geochemistry are under way by Myers and Sood,while Falster will continue to study mineralogy and mineral chemistry of thepegmatites in the Ninemile pluton. A comprehensive genetic—petrological modelis being developed, and should be published within the next year. It is hopedthat the reader/participant may find a research project of interest in thisrelatively well exposed part of the Precambrian shield of Wisconsin.

P. Myers wishes to express his gratitude to the Wisconsin Geological andNatural History Survey for sponsoring field work during the summers of 1971-1976 with Gene LaBerge in Marathon County. The Survey has also provided manythin sections and the assistance of the Survey Staff as consultants.

Another problem concerns two qua r t z monzoni t e porphyry p1 ugs (See F igu re 5) . These smal l p l u g - l i k e bodies c o n s i s t o f a l i g h t - c o l o r e d , a p h a n i t i c m a t r i x and l a r g e K- fe ldspar phenocrysts showing p a r t i a l f ragmenta t ion and r e s o r p t i o n . Qua r t z phenocrysts a r e g r a n u l a r and anhedral . These p l u t o n s a re s t r u n g o u t a l o n g a NW-SE a x i s across Marathon County, and were i n t r u d e d th rough pos t - syen i t e f a u l t s . They a r e i n t e r p r e t e d as be ing t h e " l a s t gasp1' o f t h e N inemi le g r a n i t e empl acement episode.

Whole r ock and t r a c e element geochemistry a re under way by Myers and Sood, w h i l e F a l s t e r w i l l con t i nue t o s t udy minera logy and m ine ra l chem is t r y o f t h e pegmat i tes i n t h e N inemi le p l u ton . A comprehensive g e n e t i c - p e t r o l o g i c a l model i s be ing developed, and shou ld be pub l i shed w i t h i n t h e n e x t year . It i s hoped t h a t t h e r e a d e r / p a r t i c i p a n t may f i n d a research p r o j e c t o f i n t e r e s t i n t h i s r e l a t i v e l y w e l l exposed p a r t o f t h e Precambrian s h i e l d o f Wisconsin.

P. Myers wishes t o express h i s g r a t i t u d e t o t h e Wisconsin Geolog ica l and Na tu ra l H i s t o r y Survey f o r sponsor ing f i e l d work d u r i n g t h e summers o f 1971- 1976 w i t h Gene LaBerge i n Marathon County. The Survey has a l s o p rov i ded many t h i n s e c t i o n s and t h e ass i s t ance o f t h e Survey S t a f f as consu l t an t s .

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REFERENCES

Anderson, J.L., 1980, Mineral equilibria and crystallization conditions inthe Late Precambrian rapakivi massif, Wisconsin, American Journal of Sciencev. 280, P. 289—332.

Anderson, J.L., 1983, Proterozolc anorogenic granite plutonism of NorthAmerica, Geological Society of America Memoir 161, p. 133—154.

Anderson, J.L., and Cullers, R.L., 1978, Geochemistry and evolution of theWolf River batholith, a Late Precambrian rapakivi massif in northern Wisconsin,Precambrian Research, v. 7, p. 287-324.

Anderson, J.L., Cullers, R.L., and Van Schmus, W.R., 1980, AnorogenicmetaluminouS and peraluminous granite plutonism in the mid-Proterozoic ofWisConSin, U.S.A., Contributions to Mineralogy and Petrology, v. 74, p. 311—328.

Bailey, D.K., and Schairer, J.F., 1966. The system Na 0 - Al 0 -Fe203 - Sj09 at 1 atms., and the petrogenesis of alkaline 2rocks.2 3Journal of Petrology. V.7, p. 114—170.

Berlin, L.A., and Sood, M.K. (1979). Alkaline rocks of the Stettinarea, Wisconsin Geol. Soc. Am., V. 11, No. 5, p. 225-226.

Barker, D.S. , 1974. 'Alkaline rocks of North America in the AlkalineRocks. Sorensen, editor. New York: John Wiley and Sons, p. 160—171.

Bowen, N.L., 1928. The Evolution of the Igneous Rocks. New York:Dover Publications, INc., p. 332.

Bowen, N.L. , 1945. Phase equilibria bearing on the origins and differ-entiations of alkaline rocks. Am. J. Sci., V. 243, A., p. 75-89.

Burke, K., and Dewey, J.F., 1973, Plume—generated triple junctions: keyindicators in applying plate tectonics to old rocks, Journal of Geology,v. 81, p. 406—433.

Chase, C.G., and Gilmer, T.H. , 1973, Precambrian plate tectonics — the mid—continent gravity high, Earth and Planetary Science Letters, v. 21, p. 70—78.

V. 21, 87:118.Origin of alkaline rocks. Geol. Soc. Am. Bull.,

Deer, W.A., Howie, R.A., and Zussman, J., 1963. Rock Forming Minerals.V. 2—4. New York: John Wiley and Sons.

Dutton, D:E., and Bradley, R.E., 1970. Litholo.gic geophysical andmineral commodity maps of Precambrian rocks in Wisconsin. U.S.G.S. Misc.

Emmons, RC., and Snyder, F.C., 1944. A structural sutdy of the Wausauarea. Wisconsin Geological and Natural History survey, unpub. report.

Geoll953. Selected Petrogenic Relationships of Plagioclase.

Faister, A.L. , Mineralogy of pegmatites in the Wausau pluton, Marathon County.Wisconsin, 30th Annual Institute on Lake Superior Geology, abs.

Fudali, R.F., 1963. Experimental studies bearing on the origin ofpseudoleucite and associated problems of alkali rock systems. Bull. Geol.Soc. Amer., V. 74, p. 110.

Geisse, Elaine, 1951. The petrography of the syenites, nepheline syenites,and related rocks west of Wausau, Wisconsin. M.A. thesis, Smith collene.

Hamilton, D.L., and MacKenzie, W.S., 1960. Uepheline solid solutionsin the system NaAlSiO4 - KA1SiO4 — Sb2. J. Petrology, V. 1, p. 56—72.

Hamilton, D.L., and MacKenzie, W.S., 1965. Phase-equilibrium studiesin the system NaAlSiOa (nepheline) — KA1S1O4 (kalsilite) — S102 —H2). Mm.Mag., V. 34, p. 215-231.

Hayama, Y. , 1959. Some considerations on the color of biotite and itsrelation to metamorphism. Jour. Geol. Soc. Japan, V. 65, p. 21.

Henderson, J.R., Tyson, N.S., and Page, J.R., Aeromagnetic Map of theWausau area, Wisconsin, U.S.G.S. Geophysical Investigations Map Gp-401, 1963.

Hyndman, D.W., 1972. Petrology of Igneous and Metamorphic Rocks.New York: McGraw—Hill Book Co., p. 533.

Jones, J.B., Nesbitt, R.W. , and Slade, P.6., 1969. The determinationof the orthoclase content of homogenized alkali feldspar using 201 x-raymethod. Mm. Nag., V. 37, p. 489—496.

REFERENCES

Anderson, J.L., 1980, Mineral e q u i l i b r i a and c rys ta l1 i z a t i o n condi t ions i n the Late Precambrian r a p a k i v i massif , Wisconsin, American Journal o f Science v. 280, p. 289-332.

Anderson, J.L., 1983, Proterozoic anorogenic g r a n i t e p l utonism o f North America, Geological Society o f America Memoir 161, p. 133-154.

Anderson, J.L., and Cul lers , R.L., 1978, Geochemistry and evo lu t ion o f the Wolf River b a t h o l i t h , a Late .Precambrian rapak iv i massi f i n nor thern Wisconsin, Precambrian Research, v. 7, p. 287-324.

Anderson, J.L., Cul lers , R.L., and Van Schmus, W.R., 1980, Anorogenic metaluminous and peraluminous g ran i te p lu tonism i n the mid-ProteroZoiC O f Wisconsin, U.S.A., Contr ibut ions t o Mineralogy and Petrology, v. 74, p. 311-328.

Bai ley, D.K., and Schai rer , J.F., 1966. The system Na20 - A1203 - Fe203 - SiO a t 1 atms., and the petrogenesis o f a l k a l i n e rocks. Journal o f 6et ro logy. V.7, p. 114-170.

Ber l i n , L.A., and Sood, M.K. (1979). A l k a l i n e rocks o f the S t e t t i n area, Wisconsin Geol. SOC. Am., V. 11, No. 5, p. 225-226.

Barker, D.S., 1974. "A1 ka l i n e rocks o f Nor th America' i n the A1 k a l i n e Rocks. Sorensen, e d i t o r . New York: John M i ley and Sons, p. 160-171.

Bowen, N.\., 1928. The Evo lu t ion o f the Igneous Rocks. New York: Dover Publ icat ions, INc., p. 332.

Bowen, N.L., 1945. Phase e q u i l i b r i a bear ing on the o r i g i n s and d i f f e r - e n t i a t i o n s o f a l k a l i n e rocks. Am. J. Sci. , V . 243, A., p. 75-89.

Burke, K., and Dewey, J.F., 1973, Plume-generated t r i p l e junct ions: key i n d i c a t o r s i n apply ing p l a t e tec ton ics t o o l d rocks, Journal o f Geology, v. 81, p. 406-433.

Chase, C.G., and Gilmer, T.H., 1973, Precambrian p l a t e tec ton ics - the mid- con t inen t g r a v i t y high, Ear th and Planetary Science Le t te rs , v. 21, p. 70-78.

Daly, R.A., 1910. O r i g i n o f a l k a l i n e rocks. Geol. SOC. Am. B u l l . , V . 21, p. 87-118.

Deer, W.A.. Howie, R.A., and Zussmn, J . , 1963. Rock Forming R ine ra ls . V. 2-4. New York: John Wiley and Sons.

Dutton, D.E., and Bradley, R.E., 1970. Litho1o.gic geophysical and minera l c o m o d i t y maps of Precambrian rocks i n Wisconsin. U.S.G.S. Misc, Inv . Map 1-631, p. 15.

Emmons, R.C., and Snyder, F.C., 1944. A s t r u c t u r a l sutd,y o f the Wausau area: Nisconsin Geological and Natura l H i s t o r y survey, unpub. r e p o r t .

Emmons, R.C., 1953. Selected Petrogenic Relat ionships o f P lagioc lase. Geol. SOC. Am. Mem . V. 52, p. 142.

Fa ls te r , A.L., Mineralogy o f pegmatites i n the Wausau pluton, Marathon County. Wisconsin, 30th Annual I n s t i t u t e on Lake Superior Geology, abs.

Fudal i , R.F., 1963. Experimental s tud ies bear ing on the o r i g i n o f pseudoleuci t e and associated problems o f a l k a l i rock systems. B u l l . Geol SOC. Amer., V. 74, p. 110.

Geisse, Elaine, 1951. The petrography o f the syenites, nepheline syenites, and r e l a t e d rocks west o f Wausau, Wisconsin. Y.A. thes is , Smith co l leae.

Hamilton, D.L., and MacKenzie, W.S., 1960. Nepheline s o l i d so lu t ions i n the system NaAlSi04 - KAlSi04 - Si02. J. Petrology, V . 1, p. 56-72.

Hami 1 ton, D.L., and MacKenzie, N.S., 1965. Phase-equil i b r i u m studies i n the system NaAlSiO (nephel ine) - KAlSi04 ( k a l s i l i t e ) - Si02 - H ~ ) . Min. Mag., V. 34, p. 215-2!1.

Hayam, Y., 1959. Some considerat ions on t h e c o l o r o f b i o t i t e and i t s r e l a t i o n t o metamorphism. Jour. Geol. SOC. Japan, V. 65, p. 21.

Hyndman, D.W., 1972. Petro logy o f Igneous and Metamorphic ~ o c k s . New York: McGraw-Hill Book Co., p. 533.

Jones, J.B., Nesb i t t , R.W., and Slade, P.G., 1969. The dgterminat ion o f the or thoc lase content o f homogenized a l k a l i f e ldspar us ing 201 x-ray method. Min. Mag., V. 37, p. 489-496.

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Koeliner, S.E. 1974. The Stettin Syenite Complex, Marathon County,Wisconsin: Petrography and Mineral Chemistry of olivine, pyroxene, am-phibole, biotite, and nepheline, unpublished M.S. Thesis, University ofWisconsin — Madison.

Kogarko, L.N. and Ryabchikov, 1.0., 1961. Dependence of the contentsof halogen compounds in the gaseous phase on the chemistry of the magma.Geochemistry, V.12, p. 1195-1201.

Kuelimer, F.J., 1959. X-ray intensity measurements on perthiticmaterials, I: theoretical considerations. J. Geol., V. 67, p. 648—660.

LaBerge, G.L., 1969. Preliminary report on the geology of the nort-hern part of the Wausau East quadrange, Wisconsin. Wis. Geol. Nat. Hist.Survey Open File Report, p. 13.

LaBerge, G.L., 1971. Progress report on mapping of Precambrian geologyin Marathon County, Wisconsin. Wis. Geol. Nat. Hist. Survey Open File Report,p. 27, maps.

LaBerge, G.L., and Myers, P.E., 1972. 1971 Progress report on mappingof Precambrian geology of Marathon County, Wisconin. His. Geol. Nat. Hist.Survey Open File Report, p. 28, maps.

LaBerge, G.L., and Myers, P.E., 1973. 'Precambrian Geology of MarathonCounty', in Guidebook to Precambrian Geology of Northeastern and NorthcentralWisconsin. Wis. Geol. Nat. Hist. Survey, p.31—86.

LaBerge, G.L., and Myers, P.E., 1983, Precambrian Geology of Marathon County,Wisconsin, Wisconsin Geological and Natural History Survey, Information CircularII 45, 88 p, 2 p1.

LaBerge, G.L., and Myers, P.E., 1984, Two Early Proterozoic successions incentral Wisconsin and their tectonic significance, Geological Society of America,Bulletin, v. 95, p. 246—253.

MacKenzie, W.S., 1960. Review of some contributions of experimentalstudies to petrology. Liverpool and Manchester Geological Journal, V.2,p. 369-388.

Medaris, Jr., L.G., Anderson, J.L., and Myles, J.R. , 1973. The WolfRiver Batholith — A late precambrian rapakivi massif in northeastern Wisconsin,in Guidebook to the Precambrian Geology of Northeastern and NortheentralWisconsin. Wis. Geol. Nat. Hist. Survey, p. 9—30.

Myers, P.E., 1973. Stettin syenite pluton—wall zone', in Guidebookto the Precambrian Geology of Northeastern and Northcentral Wisconsin.His. Geol. Nat. Hist. Survey, 75—76.

Myers, P.E., The Wausau syenite of Central Wisconsin, Abs., Instituteon Lake Superior Geology, p. 42, 1976.

Myers, P.E., Cummings, M.L., and Wurdinger. S.R., Precambrian geology of theChippewa Valley, 26th Annual Institute on Lake Superior Geology, Field Trip Guide#1, 123 0.

Nockolds, S.R. 1954. Average chemical compositions of some igneousrocks. Geol. Soc. Amer. Bull., V.65, p. 1007-1032.

Sims, P.1<., 1976, Precambrian tectonics and mineral deposits, Lake Superiorregion, Economic Geology, v. 71, p. 1092—1127.

Sims, P.1<., and Peterman, Z.E., 1983, Evolution of Penokean fold belt. LakeSuperior region, and its tectonic environment, Symposium on the Proterozoic,Geological Society of America Bulletin, v. 94, p. 000-000.

Smith, J.V., 1974. Feldspar Minerals, V.1, New York: Springer-Verlag,p.627.

Sood, M.K., and Edgar, A.D., 1970. Melting relations of undersaturatedalkaline rocks. Meddelelsen Om Gronland. Bd. 181, Nr. 12, p. 41.

Sood, M.K. • and Edgar, A.D. • 1972. The system diopside—forsterite-nepheline-albite—leucite and its implication to the genesis of alkalinerocks. 24th mt. Geol. Congr. Montreal, V. 14, p. 68—74.

Sood, M.K., Platt, R.G., and Edgar, A.D., 1970. Phase relations inportions of the system diopside-nepheline--kalsilite-silica and their importancein the genesis of alkaline rocks. Can. Miner., V. 11, p. 380-394.

Sood, M.K., Myers, P.E., and Berlin, L.A., 1980, The petrology, geochemistry.and contact relations of the Stettin and Wausau syenite plutons. Central Wisconsin.26th Annual Institute on Lake Superior Geology, Field Trip Guidebook #3, 59p.

Koel lner , S.E. 1974. The S t e t t i n Syeni te Complex, Marathon County, Wisconsin: Petrography and Minera l Chemistry o f o l i v i n e , pyroxene, amphibole, b i o t i t e , and nepheline, unpublished M.S. Thesis, U n i v e r s i t y o f Wisconsin - Madison.

Kogarko, L.N. and Ryabchikov, 1 .D., 1961. Dependence o f t h e contents o f halogen compounds i n the gaseous phase on the chemist ry o f the magma. Geochemistry, V.12, p. 1195-1201.

Kuellmer, F.J., 1959. X-ray i n t e n s i t y measurements on p e r t h i t i c mater ia ls , I: t h e o r e t i c a l considerat ions. J. Geol., V. 67, p. 648-660.

LaBerge, G.L., 1969. P re l im ina ry r e p o r t on the geology o f the n o r t - hern p a r t of the Wausau East quadrange, Wisconsin. Wis. Geol. Nat. H i s t . Survey Open F i l e Report, p. 13.

LaBerge, G.L., 1971. Progress r e p o r t on mapping o f Precambrian geology i n Marathon County, -Wisconsin. Wis. Geol. Nat. H is t . Survey Open F i l e Report, p. 27, maps.

LaBerge, G.L., and Myers, P.E., 1972. 1971 Progress r e p o r t on mapping o f Precambrian geology of Marathon County, Idisconin. His . Geol. Nat. H i s t . Survey Open F i l e Report, p. 28, maps.

LaBerge, G.L., and Myers, P.E., 1973. 'Precambrian Geology o f Marathon County', i n Guidebook t o Precambrian Geology o f Northeastern and Nor thcentra l Wisconsin. Wis. Geol. Nat. H i s t . Survey, p.31-86.

LaBerge, G.L.. and Myers, P.E., 1983, Precambrian Geology o f Marathon County, Wisconsin, Wisconsin Geological and Natura l H i s t o r y Survey, In format ion C i r c u l a r ff 45, 88 p , 2 p i .

LaBerge, G.L., and Myers, P.E., 1984, Two E a r l y Proterozoic successions i n cen t ra l Wisconsin and t h e i r t e c t o n i c s ign i f i cance , Geological Society o f America, B u l l e t i n , v. 95, p. 246-253.

MacKenzie, W.S., 1960. Review o f some con t r ibu t ions o f experimental s tud ies t o pet ro logy. L i verpool and Yanchester Geological Journal , V . 2, p. 369-388.

Medaris, Jr. , L.G., Anderson, J.L., and Myles, J.R., 1973. The Wolf R ive r B a t h o l i t h - A l a t e precambrian r a p a k i v i massi f i n nor theastern Wisconsin, i n Guidebook t o t h e Precambrian Geology o f Northeastern and Nor thcentra l Wisconsin. Wis. Geol. Nat. H i s t . Survey, p. 9-30.

Myers, P.E., 1973. ' S t e t t i n syen i te p lu ton-wal l zone', i n Guidebook t o the Precambrian Geology o f Northeastern and Nor thcentra l Wisconsin. Wis. Geol. Nat. H i s t . Survey, 75-76.

Myers, P.E., The Wausau syen i te o f Centra l Wisconsin, Abs., I n s t i t u t e on Lake Superior Geology, p. 42, 1976.

Myers, P.E., Cummings, M.L., and Wurdinger, S.R., Precambrian geology o f the Chippewa Valley, 26th Annual I n s t i t u t e on Lake Superior Geology, F i e l d T r i p Guide #1, 123 D.

~ o c k o l d s , S.R. 1954. Average chemical compositions of some inneous rocks. Geol . Soc. Amer. B u l l . , V.65, p. 1007-1032. -

Sims, P.K., 1976, Precambrian tec ton ics and mineral deposi ts , Lake Superior region, Economic Geology, v. 71, p. 1092-1127.

Sims, P.K., and Peterman, Z.E., 1983, Evo lu t ion o f Penokean f o l d b e l t , Lake Superior reg ion, and i t s t e c t o n i c environment, Symposium on the Proterozoic , Geological Society o f America B u l l e t i n , v. 94, p. 000-000. --

Smith, J.V., 1974. Feldspar Minerals, V.l, New York: Springer-Verlag, p.627.

Sood, M.K., and Edgar, A.D., 1970. Me l t i ng r e l a t i o n s o f undersaturated a l k a l i n e rocks. Meddelelsen Om Gronland. Bd. 181. Nr. 12, p. 41.

Sood, M.K., and Edgar, A.D., 1972. The system d i o p s i d e - f o r s t e r i t e - nephel i n e - a l b i t e - l e u c i t e and i t s i m p l i c a t i o n t o the genesis o f a l k a l i n e rocks. 24th I n t . Geol. Congr. Montreal, V. 14, p. 68-74.

Sood, M.K., P l a t t , R.G., and Edgar, A.D., 1970. Phase r e l a t i o n s i n po r t i ons o f the system diopside-nepheline-kalsilite-silica and t h e i r importance i n the genesis o f a l k a l i n e rocks. Can. Miner., V. 11, p. 380-394.

Sood, M.K., Myers, P.E., and Ber l i n , L.A., 1980, The petro logy, geochemistry. and contact r e l a t i o n s o f the S t e t t i n and Wausau syen i te plutons. Central Wisconsin, 26th Annual I n s t i t u t e on Lake Superior Geology, F i e l d T r i p Guidebook #3. 5 9 ~ .

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Sorensen, H., 1970. Internal structures and geological setting of thethree agpaitic intrusions — Khibina and Lovozero of the Kola peninsula andIlimaussaq, South Greenland. Can. Mm., V. 10, p. 299—334.

Sorensen, H., 1974. The Alkaline rocks. New York: John Wiley andSons, p. 622.

Thorton, C.P., and Tuttle, 0.F., 1960. Chemistry of igneous rocks.I. Differentiation Index. Am. J. Sci., 258, p. 644-684.

Tilley, E.E., 1957. Problems of alkali rock genesis. Q.J. Geol. Soc.Lond., V. 113, p. 323-360.

Turner, D.S., 1948. Heavy accessory minerals and radioactive studies ofthe igneous rocks in the Wausau area: Ph.D. dissertation. Univ. of Wisconsin-Madison.

Tuttle, 0.F., and Bowen, N.L., 1958. Origin of granite in the lightof experimental studies in the system NaA1Si3O8 - KA1S13O8 - Si02 - lI20.Geol. Soc. Am. Mem., V. 74, p. 153.

Van Schmus, W. R., 1973. Chronology of Precambrian Rocks in Wisconsin,in Guidebook to the Precambrian Geology of Northeastern and NorthcentralWisconsin. Wis. Geol. Nat. Hist. Survey, p. 1—8.

Van Schmus, 1976, Early and Middle Proterozoic history of the Great Lakes area,North America, Royal Society of London Philosophical Transactions, ser. A280,no. 1298, p. 605—628.

Van Schmus, W.R., and Bickford, M.E., 1981, Proterozoic chronology and evolutionof the mid—continent region, North America: in Precambrian Plate Tectonics,Kroner, A., ed., New York Elsevier Scientific Publication Co., p. 261—296.

Van Schmus, W.R., Medaris, L.G., and Banks, P.O. 1975, 1975a, Chronology ofPrecambrian rocks in Wisconsin, I: The Wolf River batholith, a rapakivi massifapproximately 1500 m.y. old, GeologicatSociety of America Bulletin, v. 86, p. 907-914

Van Schmus, W.R., Thurman, E.M., and Peterman, Z.E., l975b, Geology and chron-ology of Precambrian rocks in Wisconsin, II: Rb-Sr data for the older rocks ineastern and central Wisconsin, Geological Society of America Bulletin v. 86,p. 1255—1265.

Wager, L.R., 1965. The form and internal structure of the alkalineKangerdlugssuaq intrusion, East Greenland. Mm. Mag., V. 34, p. 487-497.

Weidman, S., 1907. The Geology of North Central Wisconsin. Wis. Geol.Nat. Mist. Survey Bull., V. 16, p. 697.

Wright, T.L., 1968. X—ray and optical study of alkali feldspars: IIan X-ray method for determining the composition and structural state frommeasurement of 20 values for the reflections. A. Min, V. 53, p. 88—104.

Wright, T.L., and Stewart, D.B., 1968. X—ray and optical study of alkalifeldspars: II determination of composition and structural state from refinedunit-cell parameters and 2V. Am. Mm., V. 53, p. 38-87.

Sorensen, H., 1970. I n t e r n a l s t ruc tu res and geologica l s e t t i n g o f the three a g p a i t i c i n t r u s i o n s - Khibina and Lovozero o f the Kola peninsula and Ilimaussaq, South Greenland. Can. Min., V . 10, p . 299-334.

Sorensen, H., 1974. The A1 k a l i n e rocks. New York: John Wiley and Sons, p. 622.

Thorton, C.P., and T u t t l e , O.F., 1960. Chemistry o f igneous rocks. I. D i f f e r e n t i a t i o n Index. Am. J. Sci., 258, p. 644-684.

T i l l e y , E.E., 1957. Problems o f a l k a l i rock genesis. Q.J. Geol. Soc. Lond., V. 113, p. 323-360.

Turner, D.S., 1948. Heavy accessory minera ls and r a d i o a c t i v e s tudies o f the igneous rocks i n the Wausau area: Ph.D. d i sse r ta t ion . Univ. o f Wisconsin- Madison.

T u t t l e , O.F., and Bowen, N.L., 1958. O r i g i n o f g r a n i t e i n the l i g h t o f experimental s tud ies i n the system NaA1Si308 - KAlSi308 - Si02 - H20. Geol. Soc. Am. Mem., V . 74, p. 153.

Van Schmus, i n Guidebook t o Wisconsin. Wis.

W. R., 1973. 'Chronology o f Precambrian Rocks i n Wisconsin', the Precambrian Geology o f Northeastern and Nor thcentra l Geol. Nat. H is t . Survey, p. 1-8.

Van Schmus, 1976, E a r l y and Middle Proterozoic h i s t o r y o f the Great Lakes area, Nor th America, Royal Society o f London Phi losophica l Transactions, ser. A280, no. 1298, p. 605-628.

Van Schmus, W.R., and Bickford, M.E., 1981, Proterozoic chronology and evo lu t ion o f the mid-continent region, North America: i n Precambrian P la te Tectonics, Kroner, A., ed.. New York E lsev ie r ~ c i e n t i f i c ~ u b l i c a t i o n Co., p. 261-296.

Van Schmus, W.R., Medaris, L.G., and Banks, P.O. 1975, 1975a, Chronology o f Precambrian rocks i n Wisconsin, I: The Wolf River b a t h o l i t h , a rapak iv i massi f approximately 1500 m.y. old, Geological Society o f America B u l l e t i n , v. 86, p. 907-914

Van Schmus, W.R., Thurman, E.M., and Peterman, Z.E., 1975b, Geology and chron- o logy of Precambrian rocks i n Wisconsin, 11: Rb-Sr data f o r the o l d e r rocks i n eastern and cen t ra l Wisconsin, Geological Society o f America B u l l e t i n v. 86, p. 1255-1265.

Wager, L.R., 1965. The form and i n t e r n a l s t r u c t u r e o f the a l k a l i n e Kangerdlugssuaq i n t r u s i o n , East Greenland. Min. Mag., V . 34, p. 487-497.

Weidman, S., 1907. The Geology o f Nor th Centra l Wisconsin. Wis. Geol. Nat. H i s t . Survey Bul l . , V . 16, p. 697.

Wright, T.L., 1968. X-ray and o p t i c a l study o f a l k a l i fe ldspars: I 1 an X-ray method f o r determining the composit ion and s t r u c t u r a l s t a t e from measurement o f 20 values fo r the r e f l e c t i o n s . A. Min., V . 53, p. 88-104.

Wright, T.L., and Stewart, D.B., 1968. X-ray and o p t i c a l study o f a l k a l i feldspars : I 1 determinat ion o f composit ion and s t r u c t u r a l s t a t e from r e f i n e d u n i t - c e l l parameters and 2V. Am. Min., V. 53, p. 38-87.

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